best thing to upgrade?

[tsinc80697]

Currently im running :

Asus A8N-SLI Deluxe
AMD 4000+ @ 2.64g
Asus 6800GT 256mb
3 x 512mb DDR400
2 x 250gb Western Digital
1 x 300gb Maxtor (unfortunately)


I was thinking about getting a new graphics card to SLI with the 6800GT, but would that be worth it with the other hardware i've got in the PC at the minute ?
Or would it be better to take the ram out and get 2gb dual channel ?
Or upgrade the CPU to dual core ?
Just looking for the best improvement possible by only upgrading 1 or 2 things, but not by a stupidly high price :*(

Thanks

 

[tsinc80697]

I'd say my budget would be £350 max probably, might be able to stretch it more, but not too sure!

Oh , and the RAM isnt all the same make, 2 sticks are the same, and 1 is geil, all DDR400 though