many graphics cards aimed for the HTPC crowd come with low profile brackets.  you can just swap them to make the card fit slim cases.  just about any of them will be an upgrade over an onboard 6150. 

if youre on a budget, watch for rebates especially now around xmas - i picked up a 1GB geforce 220 for free after rebate,  specifically to replace a friend's onboard graphics and free up the memory it was using.  not a high performance card by a longshot, but still much better than an onboard 6100/6150.  GT 430s can be had as cheap as $20 if you keep an eye out and dont mind mailing a rebate.