If you want to play Oblivion on that Dell I highly doubt you will be able to. Low settings perhaps.
I recommend you build your own computer. Seeing AGP is dead, the X850pro SUCKS, mine shall i say the who.e X8** series from ATI sucks :classic:
Your processor is low end now. Your powersupply probably wont even be able to handle some low end card like a 6200. So that would need to be recommended. Seeing that it is a dell case you will also need to buy a new case to fit the new PSU in unless you buy one specifically for Dell. Then that means you might as well buy a new mobo and go for PCIE16. You still could probably use the 2.6Ghz processor. Most likeluy s478. But i would still recommend you get a better one..
No less then 400W psu
No less then a 6 series form nvidia or get at least the 1k series from ATI
No less then 1GB of ram
Processor if you CAN upgrade, it would be nice, but if not I guess you could possibly hold onto it longer.
MSI K8N NEO4 - 70
AMD Athlon 64 3000+ - 91
Corsair Valueselect 1GB - 81
Fortron 450W PSU - 50
eVGA 7600GT - 140
Aspire X-Dreamer II - 50
Case is just used as an example. I hate the case personally
But i mean if you want a new case just browse newegg lol
Salvage your old hdd and optical drives BAM you got a computer which can play Oblivion on med settings or high if you tweak it right
Total price 482