I have an old Abit NF7-S Rev 2, with 2 x 512MB Corsair XMS3200LLPT TwinX, but since installing Vista Home Premium, I've noticed the system seems a little slower and games suffer from major lagging, although I know the NForce 2 support is a part of the problem.
Would putting 2G dual channel in make much difference as a whole and if so, would cheap Crucial/Samsung/any other value memory be fine, or would more expensive Corsair/OCZ etc be a better bet.
Considering I'm only using a Barton 3000+, overclocking won't be that important.
According to the info the NF7-s v2 doesn't support dual channel. You can have up to 3GB of PC2700, PC3200 or PC4000 (DDR333, 400 or 500). I don't know enough to know whether make of RAM matters, but I suppose 'you get what you pay for' is as true here as anywhere.
TBH I'd trust the info on the board manufacturer's site. Odd for Crucial to get something that makes such a big difference to the PC performance so wrong though.
If you're happy with the performance of the RAM you have, I'd go for the same make, just 1GB modules instead. I suppose it depends if you think it's worth around £100/GB.
I've only just noticed that you mention NForce2. Is your graphics card an nVidia as well? A lot of your problems could just be down to the current issues with nVidia and Vista. I'd say hold off for a bit until the new drivers come out and see if things improve.
In case you are using an nVidia card, the current Vista drivers are available here: click here