It will load Windows faster, load apps faster, and in some cases appear to respond to commands faster. These are things that depend on reading code for your installed programs. It won't improve anything that is dependent on your CPU, your graphics card, your memory, or any other component. Only reads from and writes to the OS.
Min size is 60/64 Gigs, which I do not recommend.
My Min recommended size is 80/90 Gig with the recommended size being 120/124 gigs.
You have to remember that the "user" size is about 10 ->15% less than the advertized size. First you lose out because manuf use base 10 to count, Computers use Base 2 - Why begcause it make the drive SOUND larger (Same for HDDs) - marketing tool. ie a 60 gig dive is only about 56 Gigs. Second, as with HDDs you never want to fill them up. Basically very simular reason: HDDs need space so defrag can run, SSD - So Trim and Garbage collector can work their magic. So 60 quicky becomes agout 50 gigs.
For Windows 7 64 bit installation a 100 GB is required.for 32-bit 64 is enough.I told this keepin in mind that u will keep ur AV,OS and some other important soft on them.As far as Brand is concerned most are good,while some xcellent.nothin as such is crap(atleast I don't think so)
BTW u forgot to mention ur current config and the purpose of ur pc(HTPC,gaming,3D or educative purpose).