If you're looking at an SSD because you want your system to boot and start applications faster, then you should look at performance metrics in this order:
Random read IOs/sec
Read transfer rate
Random write IOs/sec
Write transfer rate
The first metric is definitely the most important one for boot/load performance. Transfer rates only matter if you're copying or using fairly large files - and even then for the way most people have their systems set up (OS on the SSD and everything else on a another drive) those large files would probably be data files located on a hard drive, so the SSD transfer rates really wouldn't be relevant.
I have win7 ultimate installed w/
avg internet security
full adobe production suite cs4
Nero suite 9
Office 2007 full
with the pagefile
plus other apps
and it took up 20.5GB
I move all temp, Documents, and downloads folder to separate HD.
Moving the pagefiles will yield at least 2gb of free space.
also remember disable hibernate, system restore and remove unwanted windows feature such as tablet, drive indexing, msn messager, IE if you using something else and windows games.
Another benefit of a larger SSD drive is longer life.
q9950 oc 3.7ghz
ocz agility 60gb
3x 640gb WD raid 0
34gb raptor for Documents
ati hd 4870 and hd 2900xt
LG hddvd, blu ray 16x dvd burner
LG 16x external burner
Liteon 16x burner