I would go for the DSi or DS Lite. The DSi XL boasts some new features such as a larger screen; however, as an avid hand-held game player, I find the size to be a little uncomfortably large for extended use. Plus, they didn’t upgrade the quality of the graphics with the larger screen, so games look kind of pixelated. The DSi XL appears to be made as an attempt by Nintendo to attract an older demographic who didn’t like looking at tiny screens. :)
I have had my DS Lite for almost four years and still play it just about every day. The only problem is that mine was one of the DS Lites that had a well-known hinge problem; after a year or so of regular use, the plastic on the hinge started cracking and now my DS is all floppy. It hasn’t become such a problem that I want to trade it in (not that I could, GameStop won’t take it), but it’s a little obnoxious. So if you’re planning on buying a used DS Lite, be wary of this problem. My system was purchased in late ‘06/early ‘07 and I’m pretty sure the problem was fixed in later models.
As for the DSi, it has some nice things like a camera and better WiFi accessibility (the DS Lite is only compatible with WEP Wifi security settings, which is the least secure mode; the DSi is compatible with WEP and WPA). The advantage of the DS Lite, however, is its backwards compatibility with GameBoy Advance games. If you have and/or enjoy older GBA games (which can still be purchased used from GameStop) and want to save a bit more money, go for the DS Lite. If you want more fun features (the camera is amusing for a while, sort of like PhotoBooth on Macs) and better WiFi options, go for the DSi. They’re both pretty affordable these days.
OR… you could wait a few months until the 3DS is released. :) Not only will you have the option of a new system (although it’s probably going to cost over $200 on release), DS Lites and DSi’s will probably drop in price significantly.
Hope this helps! Wow I’m a nerd…