With looking at the rest of the specs of the PC's I would'nt recommend going any higher than a ATI Radeon 9800Pro £134.99 click here
Have a look for a more indepth explanation on sutible graphics card here : click here
As the low specs of the machine will bottleneck the performance of the graphocs card, The setup with a ATI 9800Pro will play all the latest games but not at the highest detail settings.
32MB garphics? are you sure that not integrated to the motherboard? Dell have bad habits of not including AGP ports in their Pc's make sure you check that and also ensure that the machine has a half decent PSU.
Also 20GB Harddrive ??? Doom3 is 3GB your need to get him a new one!!!
The 256 Mb cards are not really worth the extra. There are no games on the market that require that much video memory. It is just a way for the manufacturers to sell more cards as people think 256 must be better than 128.