Our family bought an xps9000 which arrived sometime early December. We bought it with 4gb of RAM, but when I go into system or control panel it only shows 1.99gb as usable memory. The motherboard has an extremely limited BIOS so I can only look at how much I have (the BIOS only sees 2gb). The rest is marked as hardware reserved.
Also, I get crazy input lag in a couple of games I have (Red Faction Guerrilla and Killing Floor). The menus have no lag, but when the game is not paused, the lag between the inputs and movement is enormous. I believe this may be because of the low amount of memory the system sees, but I am not sure.
Here's my specs
Core i7 920 processor 4gbs of memory HD5770 windows 7
More aboutsystem issues
w\ those specs you shouldn't be getting any lag.
have you opened the case to check the ram? maybe try re-seating the ram modules. Pull them out, and put them back in to make sure they're secure.