I'd also recommend against a VIA chipset, there are better alternatives out there. As for the RAM, I think 512 MB is plenty for that system. 256 MB will be enough for now, but I'd recommend 512 MB. I think 1 GB is a waste of money from a bang/buck standpoint at this time.
If you are going to go with a P4, couple it with an Intel chipset. If that's an 800 MHz FSB part, the best value is probably the i865PE (Springdale) chipset, which is used on the Asus P4P800 line of boards. The high end 800 MHz fsb chipset is the i875 (Canterwood), which makes up the P4C800 line of Asus boards. Also still available are the "older" i845PE (533 MHz FSB) chipsets in the P4PE boards. I believe Asus allows the FSB to run at an overclocked 800 MHz effective rate on these boards now. Possibly a better, if still very new, choice would be a board using the i848P chipset, which officially supports an 800 MHz FSB. The only Asus board currently using this chipset that I know of is the P4P800S.