Yeah, I'd say go with the 6600 and 1GB RAM.. given your rez preferences... I have the 660GT and 512MB RAM in my Barton 2500+ and the thing swaps to disk like crazy... especially in HL2. Stutter galore. Luckily I'm getting another 512 stick in about a week or so. My guess is the regular 6600 would be good enough for what you want to do, and the RAM will make it smoother overall.
But that's just me. Of course, going 6600GT and upgrading RAM later definitely future-proofs you, especially if you play older games that don't require as much RAM... then later on you can upgrade the RAM when you have the need (and $$).
Hm. Guess I really didn't answer anything huh!