I too have been shopping around for a new PC (half-heartedly though). Trying to determine goals and what I want out of it, I came up with this:
To be able to totally hand-pick your components that you can overclock, and assemble them yourself, shopping at Newegg, TigerDirect, etc is the best route. The trouble with this is you kinda have to know what works with what. Pretty easy, just read carefully. Also, the OS doesn't come with it, so that's an extra cost; which can be substantial if you want Windows. If I do this, I usually make sure to get components that aren't obsolete. Yeah, you can get a good price on a mobo, but it might only be capable of running the latest of a dead line of CPU socket type. Also, don't skimp on the PSU. Buy bigger than you need so you can upgrade the CPU, video, etc down the road.
Or, you can get an HP, Dell, etc system that use OEM stuff for a decent price. These systems work as-is but offer little to no overclocking. And from experience, any upgrading has to be done carefully because of mobo support and power supply limitations. Speaking of PSUs: these systems usually come with the bare minimum needed to run these systems from what I've seen.
Or, you can go to someplace like Alienware (which apparently is now run/owned by Dell) or
IbuyPower (a place I've been looking at recently) and pick from a wole list of "aftermarket" parts. You might pay a little more, but at least everything works together guaranteed.
When shopping for the CPU, look for one that is at the sweet spot for price vs performace. This tends to be 2-3 models from the top. As long as the mobo supports the faster CPU for future upgrade later on (when the price becomes reasonable), there is no need to spend double the price for a marginally faster processor, at least to me.
As far as a MAC goes: My fiancee has one and it is pretty nice I guess. I personally don't like the way the OS works in general. Besides, it's just a pretty version of Linux anyway. It can run some games in a virtual environment, but that's just a PITA unless you like that kind of thing. You could also dual boot Windows and Linux/Mac, but that's a PITA as well. Mac-heads argue that they are better for graphics. I really don't see it because the same way overpriced (Adobe) software is available in Windows too. Mac heads also argue about the lack of viruses on the Mac. Well, I feel that's only because there are fewer of them out there and virus authors want to make the biggest effect with the largest computer base. And anyway, as long as you have a decent firewall and antivirus spyware protection, and don't do stupid things with the computer, it runs malware free. But the biggest thing to me is that there is VASTLY more software out there for Windows OS. Every time she wants to do something other than "work" stuff, we have to use my Windows PC because it actually does everything.