Windows 7 run on 512 mb ram but with lag.
I have a dell optiplex gx 280 with 3 gb ram, pentium 4 Ht 630 3.00 Ghz and nvidia geforce 210 1 gb ddr3, 250 HDD and windows 7 home premium x 86 and it's works fine. I run a lot of games ( COD MW3, Left 4 dead 2 and more)
I recomand to upgrade ram of 2...