Drop in a new GPU, hmm. That's a thought. I looked at upgrading my CPU, but the whole form factor has changed. I can't even get upgrade CPUs for my MOBO. I can get a new GPU, you're right, but won't my CPU bottleneck my system anyway?
If keeping my Opteron 165 and pairing that with a mightier GPU will really give me a big performance boost, I may do it instead of building a new system. I usually build a new one every 3 years, but my desire to pay off my house keeps interfering with my hardware desires. Dammit.
A high end GPU would bottleneck, but you could go for what today is a lower end or midrange card and it would still blow what you have now out of the water. Even something like a 250 would be a dramatic upgrade.
(as for the numbers... video card naming schemes are designed to be as confusing as possible. Particularly from Nvidia, who re-released the same 9800 under a number of different names because they simply had nothing to fill in gaps for a while.)