Mac Pro late 2006 issue

Hello people.

I own a Mac Pro with 2x3 Ghz Dual Core Intel Xeon processors and 6GB 677 Mhz DDR2 FBDIMM ram.
I also has a ATI Radeon X1900 512mb graphic card and 3 Sata HDD (1tb+1tb+250gb)

I would like to upgrade "my baby" but I have a few doubts about where exactly should I start. I already asked this to a guy at an IT Store and he said he will take a look at the computer and advise me, but I'd love to hear from you on the subject.

Thanks in advance