It will be tricky, mostly since PCI graphics cards are a bitch to find these days; AGP replaced it around six years ago (my last P4 had AGP 8x, and it was built in 2004) only to fade over the last 2–3 years as PCI-E has dominated. Fortunately, that will be your biggest problem, though not your only one.
Odds are that any decent PCI card you find will be older technology, which means more heat and more power consumption. While you likely won’t have a pig like my old Radeon 9800 XT or @koanhead‘s nVidia space heater, I am not sure if your power supply is up to snuff. If your Optiplex is anything like my old Dimension 2400 then it probably isn’t unless you stick with low-end cards. But a PSU is cheap and easy to upgrade if you need to. Having fried a couple of PSUs in my day, I tend to err on the side of caution on that these days.
Cooling may be an issue, especially if you put two heat hog cards in close proximity, but like the PSU, it shouldn’t be a problem so long as you don’t get a power-pig card. Keep an eye on it.
To sum up, it is possible, and I second @koanhead‘s caveats, though I might be a little paranoid.