$2500 and 64GB (or even 128GB) might be a stretch. You also need a processor capable of crunching the numbers.
You could start with something like this:http://www.supermicro.com/products/system/4U/7046/SYS-7046A-3.cfm
You could also consider a refurbed Dell Precision. Go here to check those deals:http://www.dell.com/us/dfb/p/workstations
BTW, what makes you think a fast desktop PC with 32GB of memory won't meet your needs? What is the size of your datasets? Also what apps are you using?