If you don't have a budget then how do you know what you can aford? If you are building a computer then why don't you get the SSD now? How many games do you want to put on the drive?
I would get the SSD now and if you have to, get a regular hdd later.
Intel 520 Series Cherryville SSDSC2CW180A3K5 2.5" 180GB SATA III MLC Internal Solid State Drive (SSD)
$339.99 and free shipping.http://www.newegg.com/Product/Product.aspx?Item=N82E16820167115
This drive will hold the OS and 10 - 15 games depending on what the games are.