Using a 64-bit OS isn't really recommended for home use, at the moment. It might give more problems then pleasure. With a 32-bit OS, 4 Gb is the maximum addressable memory, so if you reconsider the OS, also reconsider the amount of RAM.
And at the moment there are few programs that make buying a quad-core CPU worthwhile. But, of course, that may change in a few years.
I'm planning on buying a new computer but I don't know a lot about PC hardware, this is what i though of (all the parts were picked by me at a "build your own computer" web page):
CPU: Intel Core 2 QUAD Q9450 2.66GHz 12MB FSB1333 Boxed (with cpu-cooler!) Socket 775
RAM: 4x A-DATA 2048MB DDR2 PC2-6400 800MHz
Graphics card: GeForce 8800GTX 768MB DDR3 TV-out DUAL DVI
Harddrive: Samsung SpinPoint HD250HJ 250GB 7200rpm 8MB SATA2
Operation system: Windows Vista Home Basic 64-bit DVD Svensk(Swedish) OEM
"starting package" including:
Cooler Master Elite 332 Miditower without PSU
Sirtec/Chieftec GPS-400AA-101A 400W ATX 2.0 12V, 120mm fan, 20/24-pin (AMD&P4 OK)
Samsung SH-S203D/BEBN DVD