The current BIOS for your machine (A17) can be downloaded here: https://dl.dell.com/FOLDER01514371M/1/T3500A17.exe
The .EXE file is for Windows, so all you do is run it and it will upgrade your BIOS without going through a lot of steps. If your BIOS is already A17 then you're already have the latest BIOS.
As for CPU, the highest CPU for your machine would be a XEON X5690 @3.46Ghz and 6 cores/12 threads. Depending on where you look they can be had for around $80 as of this date.