For capturing, the amount of RAM isn't really important. The important issue is simply having enough RAM to power the system, without making it use up virtual RAM (hard drive swap space), which causes the drives to thrash (I/O use), which in turns bottlenecks the system and causes dropped frames when the system is in full use. Honestly, 512MB can be enough. But 1GB is safer, a better solution. Most systems I have are 2GB, but admittedly much of that is just idle RAM during capturing. The RAM is more useful when those computers are used for other tasks.
If and when I need RAM, I generally buy sticks of Kingston off of Amazon
That's where I've found the best memory deals: http://www.amazon.com/s?ie=UTF8&x=0&...reative=390957