Sony's site has info on adding a hard drive:
http://esupport.sony.com/perl/model-...l?mdl=PCVRX650
http://esupport.sony.com/perl/model-faq.pl?mdl=PCVRX650
Your AGP slot is 4X meaning you could use almost any AGP 8X 4X card. The 8X functionality will just set itself to a 4X speed. That is the limit of your motherboard.
As seen in this thread your Power Supply also comes into question as you go to high end cards:
http://p200.ezboard.com/funofficials...cID=1679.topic
ATI is a good site to browse possibilities:
http://www.ati.com/
The basis of your question depends pretty much on what you want to do?
Why are you currently dissatisfied? What would you hope for?
Are you interested in building/upgrading computers or do you just want to have it done?
Are you interested the latest and fastest or just something that works solid for most things?
Are you running high end games and doing video editing? etc. etc. etc.