Mac Pro screen issue

Hi all,
my mac pro shows the apple logo at start up and the users available.
Then, either the screen goes crazy with lots of moving thin grey lines or I get to
login in under my account and then the finder has lots of artifacting on the screen
- black blocks in the finder window or parts of top bar displays overlapping text. I've
had some odd black triangle across some of the finder windows too.
If I log in to the guest user (safari only mode) the screen is fine.
Is this a graphics card issue or something else?
p.s. I have tried installing 10.8 but alas, my mac is just one model out from being suported.
Mac pro
2x 2.66 GHx Dual-Core Interl Xeon

The 7300 as I said could be on its last legs also.
10.6.3 DVD is your best bet probably.
And it isn't that hard to setup and install and be back up and running.
But I would invest in a ATI 5770 regardless, you have 6 yr old system
ATI Radeon 5770 or Nvidia 8800GT 770.html
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