From what I've read and been told, the current Samsung drives perform better than current OCZ ones. I actually just bought a Samsung 830 64GB to use as a boot drive for my old Core2Duo machine. I got it for around $64 at Microcenter. It definitely shows performance benefits (mainly in terms of shutdown and sleep mode times) but it's probably being bottlenecked by other components of my system (only using 2GB of ram on a 64-bit version of Win7).
If it's just the OS and a few of your most commonly used, every day programs, you should be able to get by with a small 64GB drive. However, if you're looking to port games you're currently playing to the ssd, then you'll definitely have to go for something around the 120GB or larger range.
From my experience, just installing the Win7 64-bit os, took 20 gigs. It's not uncommon these days for games to take up 30 GB or more. Taking into account updates for both the os and game, the total space needed will be well over 64 GB.
So overall, if the price is right, go for a 120 GB Samsung model.