Wouldn't it make more sense to have a seperate gaming system just for the sake of not having problems?
Other than that I don't see a problem. I kinda did the same. I got a htpc which also acts as server and its not problem to play a full hd movie and still manage the pc from the other account and I only got a amd 4450e. I had server 2003 running on it and even got all media center software (mediaportal) to work but the only thing I couldn't get working was the windows media center remote controller receiver. Though now i'm just running windows 7. So no as long as you don't want gaming thing will work just fine. Make sure you get a fast HDD though. Noticed that that can really kill performance more than anything else for just office work/browsing.