if you want to spend around $500 for a hardtail pretty much any company will have the same offerings

cannondale is overpriced, but they use their own brand of secondary compononts stem/seatpost/cranks/etc... and have the budget to actually do R&D on them, so they are pretty decent

specialized is a little overpriced, also have their own brand and their own R&D, i would spend my money here

gary fischer was bought out by trek, and i will never own a trek, secondary components, including wheels are usually a cheaped out option, but there are plenty of nice bikes if you like the brand

what kind of frame is your current bike? metal? size?
because honestly it may well be a better frame than what you would get with $500 these days

also consider things like sram vs. shimano
clipless pedals
fork brand
steel vs al vs cf vs ti vs ...

i would buy a good used frame and throw the rest of my money at wheels and pick up used compnents as i found them available to upgrade