Well, you haven't told me your budget and judging by choice of old Dell PC's i assumed that you might be using a decommissioned dell supplier.
Try to avoid anything with Core 2 as it is past it's glory days.
Considering that you want to play BF3, you need something with at least i3 or i5.
Try to find an Optiplex 790, 3010, 7010, 9010.
You can fit a standard PSU in most Dell towers and then upgrade GPU.
Sir I live in pakistan and my english is not so good if i go for HIS HD 6790 single or HD 6770 crossfire can dell t3400 PSU 525W handle it. But as you mentioned above dell optiplex family these system are very expensive in Pakistan and these systems are out of my budget. Ok i want to tell you, i am not high end gamer, i like mid range games like car racing games GTA 4, NFS editions NFS most wanted 2012 etc. Please give some solution to make decent dell t3400.