OK, if I had only one recommendation, I'd say go for the ASUS P4P800 Deluxe. It just blows the Abit out of the water for stability, reliability, and features.
I don't know where you're planning on ordering it, but NewEgg.com has it for $119.99 US.
Don't look at 1GB as overkill. It will just give you some overhead in the future(and in the present). It will help with things like game installing, running and it helps by keeping the OS in RAM instead of having to cache alot of stuff and with 1GB you don't have to worry about a paging file.

But, it's really just up to you, 512MB will still be nice. And with the speed of the rest of the system, you could get 512MB now, and then if you need more, just bump it up to 1GB.
But, remember... Asus!
Dave
EDIT: Re-read your post. I'd say that if it comes down to either 1GB RAM with Abit or 512 MB RAM with Asus. Get the ASUS. You won't be dissapointed, and it will save you headaches and problems.
EDIT2: I'd either get the 3.0Ghz P4 with
1MB cache, or just get the 3.2Ghz with 512K cache. Don't sacrifice Ghz along with cache.