I disagree with krone6 for the OS only bit. You can build a system around a 120GB SSD (about $214 with the model I have in mind) and still have room for a couple of games. Most of them won't matter too much when you compare read/write and IO speeds with some of the fastest platter based hard drives out there. That being said, you can totally expect to be able to install Windows 7 and a few apps (games too) on those 120GB SSDs, and then install everything else (and store your personal data) on a separate drive.
My laptop at home has one of the Corsair Force 120GB drives and it's got a couple games on it (WoW, Sup Com, and Rift) and it's still got a few gigabytes free. My work PC here has the same SSD as the boot drive, and a Western Digital 750 GB for everything else that I don't install to the SSD, and the SSD has about 78.4GB free - I even cloned it from the old Intel 40GB SSD I had been using.
Now if you cheap out and get a low capacity SSD, krone6's feedback is entirely valid, but even with a $200 SSD, you can totally swing another $50 for a large 640+GB hard drive and still come in under budget. It's getting good now. Will get better later.