The 9500GT comes in a full 1Gb variety too click here (that's a nice passive/silent one).
Download & run the free SIW click here. What does it say under Hardware > Video ?
Check that your 4Gb RAM is made by Corsair as advertised (will need to open case - check that this doesn't invalidate your warranty). Also check anything else you purchased/customised to the system matches the make/spec/quality as advertised.
When there's a shortage of the advertised spec, PC companies usually substitute components of 'equal or greater value' - which is fine when it is actually done. But I've heard of Cube247 (& some others) substituting some internal components with inferior make/spec/quality.
Contacting them & their T&C click here. G