If all you're runnign is Oblivion then just get something like this

http://www.newegg.com/Product/Product.aspx...N82E16814162019

$30 after rebate

And unlike the 9600 it doesn't require extra PSU power (since usually PSUs supplied by manufacturers are very weak)

 

can you give us more computer specs?
like make and model? and PSU, I have a felling the PSU in there is only about 250-300w... which will require an upgrade with pretty much any card

 

If you're going to upgrade your PSU, be careful, because some PC manufacturers use proprietary PSUs and putting a standard off-the-shelf one in there might fry your motherboard (I think they switch the pins around or something). I don't know if HP/Compaq does this, I know Dell used to do this (I think they quit but I'm not sure), so you might want to check before buying anything.

 

Make sure not to buy a discount ATi card if you want to play Oblivion.
I've got the same processor as you, 4 gigs of RAM, and an ATi HD3650,
and Oblivion lags like hell. It's retarded though, because Fallout 3 works fine on the highest settings.

From my experience, ATi cards don't work well with Oblivion. Stick with Nvidia.

 

With no power connector your options would be a 9500 or a 4670. For this generation, ATI has the more efficient card and thus their 6 series still works without a power connector while Nvidia's equivalent, the 9600 does. The 4670 is significantly more powerful then the 9500 and thus you should look into just how much oblivion favors nvidia cards.

 

you have to factor PSU efficiency, due to its age likely dropped so there is no way it can even deliver 300w
on top of that the PSU needs to be able to push enough amperage to power the card (I couldnt find the exact specs besides the wattage) it needs about 19amps or so

it might run or it might not, from what ive read some people got it to run on a 300w while others didnt due to the amperage problem,
its just not a good idea to run it on that PSU, a PSU calculator came about 265w for that setup

as of this point you should open up your case and look at the PSU, there should be a sticker with all the PSU specs on it and post them here if you can
a PSU upgrade is gonna cost $50 so your buget it gonna get torn up if thats the case


ive learned this all long ago when trying to upgrade my vid card in a dimension 4550, with only a 250w PSU and very low amperage, after lots of forum posts (different forum) I just decided to save up money and just build a computer rather then spending $90 on a PSU upgrade (custom made for dell cases and parts) and another $200 for an older vid card (3850 AGP 8x), it was gonna create a huge bottleneck anyway...