Rufudufus said:
Not familiar with the bike, so I don't know if it's worth upgrading. You might be better off saving your pennies for a new ride. If you like the bike, a new shock will definitely improve it. If you're willing to take a chance, you can find some nice deals on ebay under $150, sometimes much less. Check the reviews on this site to get an idea of what to look for.
Dude... the NRS is one of the best Full Suspension bikes ever.... and it's worth upgrading. The 3 was the lowest model from '03 I guess and it's basically the same frame as the 2000+ bucks NRS'. The difference between the NRS line were given by componentry. Pretty much as Giant does nowadays with their Reign/Trance/Faith lines. That's the beauty of Giants. They give you an excellent frame for the money.
Get a Manitou Black on close out at 100m travel. The ones with Lock-out have a nice damping system.
If you want to really upgrade, the best option for you are the RS Reba's. Under 400 and will match perfectly the NRS. Even the most basic model of Reba has all the bells and whistles of more expensive forks.
You're available selection is very short though because of the short travel the NRS was designed for. Max 100mm travel up front.
But I think your frame is definitively upgrade worthy.
Edit---- Another candidate is the RS SID. There are some for cheap out there, now the REBA is taking over. Perfect for the NRS but be warned... it's the lightest and the flexiest fork on earth. Only 80mm travel.