Because I was in need of Office 2013 I decided to give Tiny7 Rev. 01 (a hacked-away severely slimmed down version of Windows 7 that fits on a single CD) a go on my 600X.
Specs are 650MHz PIII, 576MB PC-133 RAM, 30GB 5400 RPM drive.
Install didn't take overly long. However, the entire time (including the half-hour I used it), the fan was barely on. This led to the bottom of the 600X becoming uncomfortably hot, almost to the point of being too hot for my bare skin. Definitely a huge minus, but fan control would probably be manageable with some driver black-magic.
The processor was at 100% for most of the time, only dropping down to 15-20% when idle. RAM usage was 230MB at idle and 320MB when changing settings and poking around the drive.
It was a usable install, but browsing (IE was unbearably slow and caused a ton of slowdown. I didn't even attempt Office 2013 (IMHO the only reason to go for Tiny7 over XP on this machine).
Comparing this to my moderately-optimized XP SP3 install, it was nowhere near as snappy or light, even under usage XP was better than Tiny7 at idle.
I'm sad to say that my 600X just isn't cut out for Tiny7, and Office 2013 is just out of my reach on this machine.
That being said, I did try 7 RTM last summer and I recall it being *****Expletives removed by Moderator***** near unusable, so while Tiny7 is an unbelievably huge improvement (props to eXPerience for his great mod!), it just doesn't hold a candle to XP (which is understandable )

On a sidenote, I've also tried out TinyXP on the 600X, and while it's crazy light on RAM (60-80MB at idle), there was little noticeable improvement on responsiveness. Running a full system yu'd probably notice a bit more improvement, though. For me, I'd rather have the compatibility and features of a full SP3.