When I first read your post I said "WHAT!!"..a Wal-Mart bike? Well, if it’s doing what it needs to on the trails you’re riding it on then keep it until you really do want to upgrade. I purchased an 2009 Specialized Hardrock XC with discs and it works great for me. It all depends on what you do. I mostly ride single track, out and back, fire roads, etc…not really doing downhill since I would need to upgrade to a more sturdy full suspension. Craigslist has some really good deals on there but you gotta catch them. I’ve found amazing deals on really good bikes from all over my area. Good luck and keep on riding!!
Once I heard your topic, I was like, "finally someone with a topic similar to mine". I just bought my first real mountain bike three months ago. I purchased a Specialized HRXC (Cross country), with full discs. It’s hella fun hittin’ some trails. I also, just recently purchased some bar ends which really helped with my last trail at Folsom. Good luck and hope to hear back.