I've found x64 to be smoother, but I'm a power user and do a lot of stuff most people don't do. I've always been a fan of more ram. If you're not using all 4 gigs, might as well, right?
I've found drivers to be very very solid on win 7 or vista x64 (since both are are 95% interchangeable, I mention both). I have had no issue with software or hardware compatibility in any way.
I'd go x64. Actually, I did with both vista and win 7. I never looked back.
As for clean install, everything will run better, plus you can tweak everything from a stock OS to get the most performance out of your machine. How long does it really take to reinstall everything anyways?
I'd go x64.
Edit: Depending on how much ram you eventually want to have, if you can drop another 4 gigs in there and that won't break the bank, you can run without a page file and see a performance boost. I've never used more than 4gigs of ram unless I'm benchmarking.
: x200 p8600 2.4ghz (probably won't OC) / 2x2gig ddr3 1066 / HDD = ? / WIN 7 (adding OSX & Ubuntu later)Desktop
: MSI K9A2 Plat / PII 945 @ 3.75ghz / G. Skill 2x2gig ddr2 1000 / 120gb ocz agility / gtx260 core 216 / IBM trackpoint keyboard