A lot of it depends on what kind of riding you're going to be doing. The 3700 will be fine for a lot of people, then again the 4500 won't come close to handling what others ride on a regular basis.

I can tell you that I bought a 4300 (non-disc) about two months ago, and I have already outgrown it. I've been doing a lot of XC since that is what we have here in Florida. So far I bought Panaracer Fire XC tires which made all the difference in the world. I also outgrew the fork and just had a new one installed a couple days ago. I picked up an 105mm '04 Marzocchi MX Comp ETA to replace the stock 80mm RST Gila T5 which has made all the difference in the world. Then again, it cost almost as much as the stock bicycle, but now it's one heck of a setup for what I do on it.

The next thing to go will be the seatpost and I may eventually swap out the v-brakes for disc since we have loads of water we run through. Otherwise the v-brakes hold up just fine for the trails we have here. The rest of the components seem to work well for me. They aren't the best in the world and are already showing signs of wear, but they do their job.

I'm still relatively new, but I know if you tell everyone what kind of riding you do now, and what kind of plans and goals you have, the more experienced folks here can let you know what your best options are and where to put your money.