i've had mine for a few days now and i have been really impressed with what this little machine can do. no, it doesn't max out bf3 at 60+ fps, but it does pretty good with no aa/af/hbao and medium/high detail settings (even in 64-player maps) at 1080p. batman: ac is very enjoyable at 1080p with the eye candy turned down/off. for the most part, you can still leave the detail settings at relatively high levels with the aa/af/hbao off. pretty much all newer games play fine at 1080p (between 40 and 60fps). is this an enthusiast pc? heck no. but it will make for an incredible lan and/or college pc. another great selling point is that you CAN upgrade all the components in this pc as long as you keep the power consumption down.
i read a review last night that some guy tested his x51 with a gigabyte factory overclocked gtx 560 ti (roughly a 170w tdp card) and it worked flawlessly (he had the 330 watt power brick). the performance increase was rather large too. now, i am certain the last thing someone wants to do right after they buy a new pc is have to upgrade it. but, since amd's new cards are out, and kepler is soon to arrive, a 560 ti could be had on the cheap pretty soon, if not right now (used). also, you could sell the existing gt 545 or gtx 555 to help off-set the cost. one of those would make a decent htpc card or extra physx card for someone.
i am currently in the process of integrating an amd 7950 into the one i have currently to see what kind of performance i can get out of this machine. i have sized it up and it will be snug, but should fit. also, the power consumption is so low that i should be able to run the 7950 at 1000mhz with no problems. i'll make sure to post back when i get the card and set it up. and ps, i didn't buy the 7950 just for this machine. i bought it for my main rig and am going to try it out in the x51.
also, i have been able to overclock the factory gtx 555 up to 900mhz core on stock voltage so far. there isn't any way to adjust voltage since msi ab doesn't support this gpu and it is a custom driver.