Woohoo! My P4 with the 768MB DDR of RAM can play 720p videos with the help of my 8800GT.
I tried running SCII on my P4, and it does not run at all... So I guess it can only play games from 2005 and below.
If only I can find an AM2 motherboard for cheap ($50 was the lowest). Then I would have an 64 X2 4800+ with 1GB of DDR2 RAM.
I also bought L4D2, since it was $5. I also bought one for my friend since one of the "Steam Scavenger" was to gift a game.