You should be able to sell the nf7-s and xp2500 and get close to £100 for them on ebay; i sold my nf7-s v2 the other week for £50 and my xp2100 for about £35, so as your processor is a barton i presume it is most likely a good clocker so u should get a bit extra for it.
so that should increase your spending to about £300.
Get the asrock 939dual-sata2 board, it accepts both agp and pci-e gfx cards, its a great mobo and can be had for about £40 online.
For a CPU i would recommend an opteron 144/146, they can be had for about £100 -£130 if you look hard enough, but it is probably worth it if u can stretch to the 146. Overclock this baby and u will have a chip that performs very close to an FX55 / FX57.
So that should set u back about £170 max. Now what u do for graphics is your choice...you can use your 9800pro for a little longer and save up a bit more and then sell it and get something like a 7900GT on pci express; i dont know too much about new gfx cards as im still using my 6800nu from when it was first released.
However i have a an opteron 144 running at 2.5Ghz with the asrock and it is a great combination for a student just like u and me! its a great system on a budget!
EDIT: forgot that you need some more ram! add in another 512 or a 1GB of the geil stuff too!! i guess that changes thing, but definately go for an opteron with the asrock board to start with!
