Upgrade

[Xyphos]

...

I would probably see if I could get a couple more years out of it and then start thinking about rebuilding. Maybe throw 40-50$ on ram to get you closer to 8GB...

This. I'd say go for another year or two. Prices for Sandy bridges will drop once the Ivey Bridge comes out anyway (Unless you just want to upgrade to an Ivey, in which case you should still hold off).

If you really want an upgrade, do what Snowreap said; the difference between an HDD and an SSD is like night and day.