BioShock. Crysis. Call of Duty 4. These and other suh-weet new games require a generous helping of video power. If your system is more than a couple years old, or you bought a budget desktop with an integrated graphics processor, you'll have to run your games at low resolutions without any of those dazzling eye-candy effects. That's assuming they'll run at all: Many of them require a video card with at least 128MB of RAM.
I recently upgraded my aging Pentium 4 box with a GeForce 8600 GT-based video card, much like the one currently available from TigerDirect: It's the XFX GeForce 8600 GT XXX with 256MB of RAM, DirectX 10 support, an SLI interface (in case you later decide to pair it with a second card for some serious graphics horsepower), and a copy of the excellent wargame Company of Heroes. You supply the PCI Express slot; it'll supply the high-res gaming goodness.
Your mileage may vary, of course, but I've been running BioShock on my widescreen monitor with nearly all the effects amped up, and it's smooth as silk. (And what a fantastic game, by the way.) The XFX card will run you $99.99 plus shipping, after a $30 mail-in rebate. Definitely a worthwhile upgrade, though gaming experts would likely suggest spending another $75 or so on the GTS version. For casual gamers like me, however, this is a plenty powerful card.