the marzocchi mx comps on the bottom of this link are good forks in your range(for xc). if you dirt jump then the marzocchi dirtjumper on the top is a good fork.
*edit, the link brings you to page one of the forks, go to page three for the forks i said above*
Personally I wouldn't bother with them as the handling of the bike will be messed with signifigantly. The Jett C even with a "normal" weight rider (160 lbs) sank through half it's travel when simply being sat on dropping it's ride axle to crown (AC) down to about 401mm. Throwing a 'Zoch on it (properly tuned for a sag of 20% of total travel) will give it a ride AC of 447.3mm (a nearly TWO INCH) difference which will most definately affect handling in an adverse fashion.
Dirt Jumping a '99 Hardrock is totally out of the question. It was designed prior to the "X-treme" movement and was barely into the suspension era. It relied on older geometries than those found on even the Rockhoppers of the day the same frame was also used with a rigid fork in the years prior.
I ran into a similar problem when I was debating on on whether or not I should revamp or retire my personal 1999 Specialized Hardrock Comp FS. I decided the most prudent course of action was to retire the ol' girl and upgrade to a trail bike of a more modern design not a low end throw back with token front suspension.
There is light at the end of the tunnel however. The Cromoly frame of the Hardrock (except for the A1 versions which are aluminum) lends itself to being a comfortable commuter / in town bike with a couple minor alterations. One of the best modifications I've made to mine was the removal of the godawful Jett C fork and replacing it with a Surly 1x1 rigid. It's AC of 413mm means that it's much closer to the actual ride height of the Jett C and has the benefit (even though it's a rigid) of being better at controlling bumps and vibration much better than the stock fork.