It depends on which e350 board you get, the cheap foxconn ones will have only a 64 bit bus width and won't allow overclocking of ram and cpu.
I have the gigabyte board which cost me $90 real cost after rebate (Mini-itx, 18 TDP form factor). The bus width is 128 bit and the ram is oced using dividers to 1333 mhz, with the fsb clocked to 110 (10% increase) it gives me ~1.8 ghz cpu and ~1500 mhz ram.
I can play fallout 3 on low settings at 1080p, I can play skyrim on low settings at 720p, halflife 2 medium settings at 1080p, mount and blade maxed out, left 4 dead low settings 1080p; all these games at 30 fps plus, skyrim with some dips.
Basically any game except battlefield 3, witcher 2 and other games in that beastly category are playable on this setup. Although I think i may be able to get bf3 working on low settings at like 640x480 lol.
Overall, I've been overly impressed with this tiny machine of mine running on a 60 watt psu. The case is a habey server chassis from newegg and can be mounted to the back of your monitor. Price/performace wise you can't beat it in this form factor.