I brought an extra 2 gig to go in mine, bluescreened all over the place.
When I eventually got Vista 64 installed (having removed the dodgey memory) it worked fine, untill the full 4 gig was installed.
Was getting nvidea blue screens, and irq not less or equal to plus numerous other errors.
Would boot vista, be fine for about 2 minutes untill I clicked on something.
Turned out one of the new sticks was dodgey, so waiting for them to replace it.
With the old memory in, it's as stable as XP ever was, and not crashed once in the last 2 weeks or so.
Try it with only the old meory in, or even just the new.
Also, try the Vista memory diagnostic on boot up, see if it locks solid on a certain stick, mine worked fine with 3 of them, and locked solid on the dodgey stick.
In XP, with the 4 gig in, it was fine (as it only recognises the 2.5 gig or so.)