I've never been able to figure out at what point your computer is too sucky to make use of a good video card.
I have a Pentium D, which makes me wonder whether I should bother upgrading at all anymore, but supposedly, if I upgrade to an E6600 Core 2 Duo (which my mobo purportedly supports), there's a massive performance upgrade along with a decent-ish, currently middling video card.
I can tell you my Q6600 Core2Quad actually delivers better gaming performance than the Core 2 Duo because I've run those two machines side by side... But even the duo is a big step up from the Pentium D.
But in my experience, what usually makes a "computer" too sucky for a video card to make up the difference is has not been the CPU but the other parts of the data pipe - the speed and amount of RAM, the bus clock, the access time and transfer rate of the hard drives. The step up from IDE to SATA was a huge one for gaming. It's made the difference in load times like you wouldn't believe... and of course, the jump from DDR to DDR2 ram was also a big one, and doubtless whatever's coming down the pipe in an iteration or two will be just as big again.[/QUOTE]
Not too concerned with hard-drive (500 GB SATA). The RAM isn't so hot, but it's workable (2GB 533Mhz DDR2 for an XP system), and while I can add more, the best I can do is 3 GB of 667Mhz DDR2 (4 if I upgrade to 7 from XP). The mobo currently has an 800Mhz FSB, but with the firmware upgrade needed to install an E6600 (can't install a quad on this sucker), it will increase to 1066Mhz.
I could upgrade all of that (+ a new video card) for maybe $400 and change ($100 for 4 1GB sticks of 667Mhz RAM, $80 for the E6600, $100 for Win 7 Home Prem Upgrade, $130-ish for an HD 5750), but at that point it may be more worth it to put it in the "new desktop fund" jar, since what comes out of this upgrade would still be far behind even a budget system today (except possibly the video card).