You are very much over-due for some new larger more efficient drives.
the most common best approach goes something like this....
SSD 120GB for system
WD Black 1-2TB for data (and put all your data and media files, all your home account sub-folders)
Backup. I use WD Green but use what you want. I have a small boot volume, TimeMachine, and 3rd that is a clone image of the boot drive volume.
Very fond of WD 10K VelociRaptor drives. Not much louder now than the WD Black 2TB, $100-200 for 250GB to 1TB.
You can use any size SATA 3.5" drive, most now are 1TB and up to 4TB.
You should have always bought Amazon or Newegg or outside of Apple, Apple can often charge 3x what those places charge.
Probably want to add or replace FBDIMMs you have, these are excellent and price in the last year has fallen.2x2GB FBDIMM DDR2 667MHz @ $29
WD Black 1TB $93
WD Green 3TB $149 - backup
WD VR 10K 250GB $103 200MB/sec boot drive :
Those last and last and make a nice boot drive.
SSD: Samsung 840 128GB
Over-due to upgrade and replace the graphic card most likely it sounds like also:
ATI Radeon 5770
Your OEM 7300GT isn't helping now and with Lion or the X1900 dust magnet and out of date too. If you have a functional 8800GT still working you are lucky and no need.