Question about Dell Precision Workstations

About 3 years ago I got a really great deal on a Dell Precision 490 WS that I couldn't pass up.  It's the fastest desktop computer I've ever had.  It really screams even with Vista running on it. 

I know that these Dell Precision systems aren't designed for playing games, but recently I got interested in playing some games again.  I've been playing Call of Duty 2 and it runs fairly well.  It originally came with an Nvidia Quadro 3450 video card in it but I upgraded to a newer card that's in the signature that was better for playing games. 

I was just wondering what is the main thing that holds these computers back as far as speed needed for playing games?  Is it mainly that it uses ECC RAM.  I've heard that this can slow memory down about 10-15%.  I know that the Nvidia Quadro isn't a gaming video card so I know that this was one of the things that was holding me back.  I've noticed that every once in a while it can stutter some when a lot is going on in a game.  Could this be the fact that it's using ECC Memory?