That SSD is sold out unfortunately. That would have been a good grab, even though it'd be bottlenecked by your mobo. For your setup, I would look into SATA II drives. I'm pretty sure socket 775 mobo's had the 3gb/s and not 6gb/s of current SATA III. That'll save you some money. It'll still operate faster than a conventional HDD.
As far as a "boot drive" goes, 60gb is sufficient, but not futureproof. Windows will never take up more than that. If you want to start loading up other programs on there, then yes, looking into 100+ is ideal.
Honestly, If I wanted a quick fix on that desktop, and I was strapped for cash, It'd be a SATA II 60g, unless the price difference between SATA II and III is minimal (which it might be, since sata iii SSD's are in a price-war it seems). That's the cheapest option.
To futureproof some, getting a 128+ gb sata iii is a good idea as Lanx was correct in its transferability to future builds. Most of the other hardware you have will be rendered obsolete if you don't want to bottleneck future generation hardware.
As far as Sata III brands go, many positive marks have been given to: Samsung 830, Crucial M4, OCZ Vertex 4 (pre 4th gen OCZ is iffy for QC reasons), Sandisk, Plextor M3, Intel 520/330 etc.
Ultimately I'd start lurking in the "build a pc sales" subreddit. people are pretty good about posting some crazy deals. For instance, getting a 128gb Sata III OCZ Agility 4 for $54.99 with free shipping for my laptop wasn't a hard deal to splurge on. And yes, it makes a huge difference for booting and normal windows applications.