william p :
The T3500 isn't a BTX ( T3400 was). But it's not ATX either. It's a modification of BTX. It actually has quite a bit of potential as is.
There are some unlocked Xeons that run in it, and the 3 channel DDR3 1333RAM can still keep up. It's popular in these 2 threads.
https://www.techpowerup.com/forums/threads/throttlestop-overclocking-desktop-pcs.235975/
https://www.techpowerup.com/forums/threads/dell-workstation-owners-club.243124/#post-3830958
OP didn't say T3500, the first reply did. Search the model number of the board its BTX, or looks like BTX as you say, doesn't matter, an ATX board isn't going to fit where it is.
And sure for some tasks an old Xeon in there can "keep up" but any processor that fits in the board he bought would eat it for lunch in gaming.