If you want a Mac Pro, go for it.
But one tip, computers dont get slower. The Operating System does. Files on the OS get moved and changed as settings get overridden. The CPU becomes slower because its trying to read settings from files that are now scattered on the partition. This happens faster if the OS is on the same partition as your documents.
All you need to do is move your files to a back up drive, and reinstall the operating system. Your computer will work like new.
You might want a new hard drive if its several years old. Wipe the old one and use it as slave drive for file storage. Even if it goes bad in a couple years, the files are almost always recoverable from a slave drive.
This is a mistake people make. Abandoning great computers because their OS needs a reinstall. Its rare that a mother board goes bad. Its the operating system first, then the harddrives. Both of which are fairly cheap to fix.
But then again, Mac has you guys by the balls owning your world. So god bless you.