Samsung 840 Pro 128GB. $140 http://www.newegg.com/Product/Product.aspx?Item=N82E168...
128GB is enough for an OS drive. After formatting, a windows install and moving the pagefile you'l have ~90GB left for programs and games. Though always leave about 10GB free on an SSD, having the drive maxed out decreases performance.
Use the extra cash to get yourself a GTX670 or HD7970, or maybe a good mouse and keyboard set.
128 is enough for most, but game installs can be quite large. If you want really fast game loads and typically have a lot of games installed at the same time, then I'd suggest 240-256GB. If you are willing to juggle games between the 7200RPM drive and the SSD then 128 is fine. Since you have a $300 budget, I'd go bigger.
I'd be looking and Plextor, Crucial, Samsung or Intel based on reliability. Right now the Plextor M5S, Samsung 830 or Crucial M4 256GB @ about $200 seem like the best bang for the buck to me.