I have Optiplex GX280 SFF (that means super small frame, matx desktop) with 160 Watt factory PSU. I upgarded cpu to pentium 4 650 (3,4 GHz, 64bit) 4GB ddr2 800MHz RAM and actualy seceond ATI graphic cards. If you find pentium 6x1 processor then its better cuz its less power consumpting due to 65nm technology (the one I have is 90nm and costs me 84 W of this insufficent power).
Graphic cards I tried are Radeon x1550 (or x1300, there is no difference) and radeon 2400 hd that burned today (probably due to fan problem, I got it with damaged power connector and it didn`t circulated - after a long hour of playing Fallout 3 it run down and definitly wont stand up). Im going to try Radeon 3450 or 4550 as soon as I put my hands on it.
Finally, anserwing your question, it can support every S775 Pentium 4 / Celereon, Celereon D
My question is, if there is possibility to change PSU to better one (250 Watt would be OK) and if GX280 can handle mentioned cards (ATI 3450 or 4550) ??