It depends on what your PC usage is. The only real reson I can see for getting a dual core processor right now is if you consecutively run multiple programs. Very little in the way of software is currently optimised for dual processors and by the time dual cores begin to be considered as a requiremnent - your x2 3800+ may well be obsolete (or at least bottom of the pile). The RAM upgrade could well be useful, it will help load times and smooth out any stutters, with Vista reportedly taking up 800mb of RAM to operate fully 2GB is going to be a minimum for any kind of decent platform to run games on if you want to upgrade your OS. Currently AMD rigs running dual channel prefer 2 sticks of RAM for optimum performance, so if it were me I'd be looking at getting a decent 2x1GB 2GB kit and selling on my old 2 x 512MB sticks. At some point you'll have to get a PCI-E motherboard and card(s) if you're not happy with your current rigs gaming performance I guess this would be the area to look at. Bare in mind that demand for top level AGP cards is high at the moment so you could potentially sell on your 6800GT for ~£120. If you could put this towards a 2GB kit (dont forget the money from your selling your 1GB kit) then you'd have a pretty decent rig with the intention of upgrading to a dual processor CPU when the time was right.
To sum up I'd buy the motherboard / graphics / memory and sell the old kit to help fund this. I'd put off buying an X2 CPU until I could actually gain some benefit from it.
