Correct me if wrong, but that looks like a slim case - only low-profile (L-P) cards will fit in this. If playing the latest games is your main reason for upgrading you'll be disappointed because L-P cards available are poor for this sort of thing.
Arguably the best L-P graphics card one could get is a HD 3650 card, e.g. click here. Couldn't find one available on a UK website though, but I'll keep looking.
Others to consider are L-P models of 8600GT, 2600XT, 2600 Pro, 7600GS, HD 3450 & 2400 Pro.
Your PSU being a 'Dell' will work with any of the above, in addition to any other L-P card that does NOT require dedicated power from the PSU - none of the above do.
They say in some rare cases the newer PCI-E 2.0 cards (HD 3450/3650) may not work in a PCI-E 1.0 slot. So choose your card, & we'll help you determine if it will or not.
Related eBay search click here - some seem grossly overpriced, so check online prices thoroughly before buying. G