make sure you buy the same ram that you already have in. dont mix and match as it will make your system unstable at the least and kill the mmu/ram and may even damage the cpu if the timings are to far out.
you will need a minimum core 2 quad q6600 if you want to make your pc a entry level system. you can pick up a q8300 which is as good for around 80 on fleebay.
the gfx card barley passes muster so if you can save a little more you will get so much more for the money... there really is a step change 1s you get up to the 7870 or better the yes its x2 the price but its more than x2 the performance.http://compare.ebay.co.uk/like/400413309117?var=lv<yp=AllFixedPriceItemTypes&var=sbar&cbt=y
q8300 £59/£80 from a very reputable seller with free shipping.