My first advice is that if you have the machine on an ethernet wire, that’s not in the way, keep the wire. A wire will always give you more reliable service and Much faster service than wireless.

Any “802.11n” standard card from a name brand will do pretty well. (3Com, Belkin, Netgear…)

The PCI slots are just fine for this. It is easy to do. You will have to remove the little panel at the back of the machine that corresponds to the slot into which you’re installing the new wireless card. That involves either a philips screwdriver (on nicer cases), or possibly having to knock out the panel if it’s not screwed in.

Yes, be careful of static electricity. Be sure to touch the metal of the case, before touching the card or installing it, to dissipate any static built up on you.

I agree with everything @dabbler said, plus…

1)
802.11n is faster than 802.11g
If your router is 802.11g, then an 802.11n card will work fine, but only at 802.11g speeds. An 802.11g card will be less expensive.
For the most purposes, you will never see a difference between 802.11g and 802.11n – Very little web content moves faster than 802.11g
The big exceptions would be moving files among computers in your house, streaming video in your house – then 802.11n is noticeably faster. I’m not sure about streaming NetFlix off the Internet – I have not tried that.

2)
Another option is external USB.
Pro: Easy install, you can use easily move it to different computers, including laptops
Con: Extra stuff hanging off your computer

3)
Newegg.com is a great place to buy computer parts. At least check out the users’ product reviews. Its customers are more experienced than average, so their reviews are very helplful. It’s not unusual to find a product with hundreds of reviews, so you can count on a reliable average score, rather than some small number of happy or unhappy users.

+1 newegg, I end up buying there more often than anywhere else recently.
A good runner-up is mwave.
And a bit less often tigerdirect They used to be at the top of the value list but are inconsistently there these days.

Personally, I have had luck with my Netgear USB dongle. Sometimes I have it in the included docking cradle, sometimes I take it off and put it on a system I am repairing. It is 802.11n, was fairly cheap, and the fact that I can quickly transfer it to any computer I am fixing also helps.

However, as @jaytkay points out, 802.11n isn’t really faster in many cases since even 802.11g is far faster than most internet connections. But it does help considerably on file transfers between computers on the same network (it’s faster than even 10/100 Ethernet) and it seems to have more range as well.

Thank you all for your help.

Do I just drop the card into one of the PCI slots, tighten some screws, and that’s it? Is that all?

Also, I believe that I have some wide PCI slots and some small slots in the computer. I think they make wireless cards for both sizes. Should I prefer one size over the other? Does size matter?

Also, I see that the cards come rated N150, N300, N600, even N900. What do the N numbers mean?

Those cards all use 802.11n (the latest and greatest) but some use multiple channels to multiply bandwidth. But you need (and probably do not have) a matching router to use that extra speed. N150 is good enough.

I would use a PCIe X1 since you have the of them for a reason.

@jaytkay and @jerv I’m tempted to just go with the USB dongle for now. I saw one advertised today for $11.
But @jerv, if I say that I would like to upgrade the router later to do home theater video and audio streaming (Netflix, Hulu, YouTube), would that change your recommendation/thoughts for the PC wireless card?

Ref: streaming media over wireless, any 802.11n connection will be plenty for those internet services. I think it was @jerv who pointed out recently that Netflix HD is not full 1080p HD, it’s780p…or 1080i both of which are reduced resolution from an HD DVD. And Hulu and YouTube are typically less resolution than that.

If you start streaming true HD from one of your other home network gadgets (another computer, a NAS) things could get more challenging over wireless.
But ‘N’ is designed with streaming media in mind, minimizing the intervals between packet to any given connection.

The USB dongle could have an extra challenge, if streaming high-bandwidth stuff, of USB bus contention with other USB devices (mouse, keyboard, webcam) but probably not.

No, because even the 150mbps of regular 802.11n is still far faster than your internet connection. Your streaming will never be faster than your ISP delivers, and most home connections are no faster than 30mbps for decent cable; my mid-tier DSL connection is 7mbps, or about one-twentieth the speed of my wifi.

Good to know, @jerv. You’re a fountain of knowledge! Does your DSL connection work well with Netflix and similar services?

Does the N number refer to the Mbps, then?

And @jaytkay, yes, if I get to the point where I might want to try video streaming, I’ll probably replace the USB dongle with a true wireless card at that point. For now, for $11 I’m willing to give the dongle a try.

Well my wife watches a lot of Hulu without interfering with me surfing on my laptop…

802.11n= 150mbps per channel
802.11g= 54mbps
But the way those particular ones are numbered, yes. That may not be true of all manufacturers, but suffice it to say that pretty much any router and wifi card/dongle on the market right now is fast enough.