Youve gone and spoilt it by including Oblivion!!!!
Read this click here
No £70 or even £100 card will do justice to oblivion....AGP or PCI-e.
The best you can do right now with your budget is an NVidia 6600GT.It wont be great on Oblivion but will eat the other games you mention alive.Not to mention being a HUGE step up from what you are currently using.
Your processor will cope and providing you have 1GB(or more) of RAM you will be fine.