I did alot of oblivion testing with that same 8800GS. It will handle Oblivion at max details far better than your Sempron 3400+ will. 1280x1024 2x/16x or 1600x1200 no fsaa are both possible throughout the game at max or near max details. With a single core CPU, you'll have slower minimum fps outside, and worse performance in towns around large numbers of NPC's. You may have to reduce some settings from max.
I would go for it, if you want COD4, it's incredible value for the money. If not for COD4, I would say go for the evga for $100 AR. It is clocked higher and has a better cooler with a variable speed fan vs the XFX constant 100% louder fan.