I made the "mistake" of buying five 30GB OCZ Agility drives for my first SSDs. I had planned on setting up two identical RAID-0 arrays on two P35 / ICH9R boards. But soon discovered that without TRIM, in a week's time, the performance of the array had degraded below that of a single drive.
So much for that idea.
So I was running them as single drives. Win7 64-bit will install, leaving about 2GB or less space, after the hibernate and pagefile on a 8GB system.
They are faster than a HDD, but not by as huge an amount as you might think.
I bought two 60GB Adata drives, and four 50GB Vertex 2 refurbs, for use as boot drives. I wouldn't go less than 50-60GB for a boot drive. For only one drive in the system, 120GB as the minimum, but prefer 240GB or larger, if you can afford it.