Typically you want to figure a windows + programs will take between 25 -> 35 gigs depending on how many and what programs you install. This is excluding games.
as rolli59 indicated the min size is 60/64 gigs (some have gone as low as 40 gigs, but I do NOT recommend). and rhe recommended size is 80 -> 128 gig. You just missed a sale on the Intel 320, an 80 gig SATA II SSD, for $80 - an oustanding drive at that price.
If you can go upto $110 then :
http://www.newegg.com/Product/Product.aspx?Item=N82E16820147133
The 120/128 gig SSDs are currently $180 and up. The agility III can often be found on sale (I have 2 of them and normally do not recommend them).
Yes, SSD price per gig will drop, but my crystal ball is Inoperative. The demand tends to keep the prices up so may be awhile.