No you cant get a cpu, mobo, ram AND gpu for £100, you could get a cpu maybe but nothing else, and then it would have to be supported by your currant mobo.
For the amount of cash you have id get another gig of ram that will help alot 1GB DDR PC3200 Memory - Cheap at Ebuyer!
You say you dont game so i wouldnt bother getting another graphics card then, and do you have enough hard drive space? if not get this Seagate ST3320620AS 320GB Hard Drive SATAII 7200RPM 16MB Cache - OEM
If you do have enough space dont bother wasting your money and get 2gigs of ram to make 3gigs in total, your pc will be much faster with another 1 gig but another 2 would make a big difference