Right, for £37.99 you can get a radeon 9550 graphics card delivered from amazon (cheaper than ebuyer since you have to pay more for ebuyer delivery)
I recently purchased it for a friend who has a 2.4ghz processor, 512mb ram and was previously running on integrated gforce 4 graphics which were more like gforce 2 standard.
The radeon 9550 is a tuned down 9600 pro, so all you need to do is uninstall the nvidia card through device manager, take it out, install the new one and install the drivers supplied on the cd.
Then head over to http://www.techpowerup.com/atitool/
and download ati tool. This is a small overclocking application which you can use to simply increase the speed of the card from 250mhz to 400 mhz straight away - or if you prefer you can let it detect the fastest speed automatically.
After doing exactly this to my friend's pc it now runs oblivion, cod2 flawlessly at 1024 * 768 with the graphics on low for oblivion and medium for Cod 2. When i say flawlessly i mean no errors or slowdowns - and it doesnt look bad either.
A budget option that should help you a bit.