I haven't used vegas in a while (I'm more of an adobe user; now trying to switch over to Avid), but i would advise you to get a a dual-core cpu (something like an intel core 2 duo), as much ram as your system can take, and a good graphics card.

You might also want to set the video monitor preview to draft mode or half-res/low-res while editing and just render your whole project as a qt mov for full res previews.