MARKMAKAVELI said:
Ok, is eBay the best place to buy those things? How much should I be looking to spend? Would you be confident that, after these upgrades, my computer would be able to run pretty much all the new releases on the pc? E.g. PES2008, FIFA08, Tony Hawks etc?
I buy stuff on ebuyer.com. I also used komplett, aria and microdirect before. I search for what I want on the sites and get the cheapest one, but I try to shop at one place at a time to keep delivery costs down!
I think the minimum you'll get away with spending for moderate performance in the games you mentioned is around £60. You do need a PCI-Express slot to use newer graphics cards. (You probably have that or the older AGP slot) as for finding out, you could look at this pic I found
http://content.answers.com/main/content/img/CDE/PCXSLOTS.GIF and compare

D LOL I'm not a computer expert either!)
If you have an AGP slot the best AGP cards are Nvidia Geforce 6/7 series which are fine for playing the games you mentioned (As I said, I played PES2008 on my 6800GT 256mb/AGP graphics card and it was fine at 1024x768, highest quality) I found 6800GS 256mb for £55.01 here
http://www.komplett.co.uk/k/ki.aspx?sku=317265, but you should look around and ask what offers the best value for money, 6800GT isn't such a significant jump from your current graphics card really. If you do have a PCI express slot your options are much larger.
I didn't notice before you had 1gb RAM already, seeing as thats what I used for PES2008 on my old machine maybe you'll be ok rather then getting another 1gb.
For a great performance I'd try to get Nvidia 7950 AGP card, or if you have a PCI express slot, the 7900GT does run PES2008 fine too. Them cards cost between £100 - 150 (BTW I know I have not mentioned any ATI graphics cards, but I don't know what ones are good in them price brackets, to make sure you get the best value you should research ATI cards too!)