How do i decide what DDR memory to buy?

Question

I have a Dell Dimension 300 and im runnin windows xp SP2, ive got 256Ram and i need more because my computers become considerably slow, ive got one more slot to fill im thinkin of buying an extra 512 to fill the slot but when i look on a site like dixons 512DDR options come up but theres different ones to chose from eventhough there all 512 i just wanted to know the difference and what to buy (what wud be the best for me)

http://www.dixons.co.uk/martprd/store/dix_page.jsp?BV_SessionID=@@@@0984162560.1167153762@@@@&BV_EngineID=ccfgaddjkedfdhgcflgceggdhhmdgmh.0&page=ProductList&category_oid=-28795&fm=3&sm=3&tm=2&show_all=true

This is the dixons page i was looking at what one should i get?

Answer 1

http://crucial.com
will scan your system and tell you which one you need

Answer 2

You need to find the speed that your motherboard supports. And if you want to run dual channel, then you might want to get the same capacities. So if you want dual channel and want a gig you will need to get 3 more 256 sticks. If you want 512 just get 1 more and put it in the corresponding banks..