On a 4850 and the rest of your gear, a 750w is top end, you could get a 600w range if it saved a few pounds.
For RAM I would get a 4GB kit (2x2GB) 800mhz stuff. Link your vendor and we can look at their choices.
I'm buying most of my gear from dabs.com, and some from amazon.co.uk. I though the following amount of memory would be suitable. What do you think?
I wouldn't wait for Win 7. You might be able to buy it in December, but do you really want to use a Microsoft operating system before SP1 is out? Win 98, XP, Vista - they were all a lot better after SP1.
I'd stick with Vista Home Premium 64-bit until then.