Vista isn't out yet, so there won't be a definitive HC list yet. However, things you can be sure to need:
- AT LEAST 2gb RAM if you want to run any 'proper' apps or game
- AT LEAST 1gb RAM just for running it stably
- Dual core is probably a good idea
- A good graphics card. My 9200 didn't cut it at all.
- At least 3ghz/3000+ processor, or equiv. Opteron etc...
But like I said, you're talking about an OS that won't be out until Jan/Feb '07 and isn't near completion yet.
Plus you have to factor in DirectX 10 support, of which there is none so far. You're much better off waiting.