The car analogy isn't accurate. Besides, people are finding out that PCIe doesn't mean you will get 16x speed. Some boards can only go 8x when in SLI mode. :? But I digress.
Anything from a 6800GS or x800 and up will be a significant improvement over your trusty, but tired 9800Pro. If there are eVGA inventory problems, I suggest looking at BFG, PNY or MSI 7800GS cards.