790 has 2nd gen CPU. 7010/9010 - 3rd gen 7020/9020 - 4th gen 3010 & 3020 aren't as expandable, but sure can be cheap.  But look out for dual core CPU's vs. quad.
mI have a dell optiplex 790 had it for half a year but I haven't messed with it In a while till last night. I found a 20yo graphics card and put it into the machine. It works!!!, but it's very limited since its age .I take it out of the machine and plug my VGA to the intagrated graphics but the computer give me a error (2,4) with two numbers glowing. I know it means graphics card failure but on intagrated graphics??? That's Tied up to the cpu and it still works just fine. Is there a fix for this mess?
dell optiplex 790 intel HD graphics not working