Amethyss
Your Mx500 will work just fine. Just plug it in... it will work. Most Mac artists get a multibutton mouse any way.
Check out the store at www.apple.com. They also have a link to factory refurbished machine that still carry the full warranty(lower left of Store page). You can also pick up a copy of MacWorld or MacAddict. Many of the vendors in the back will deal if you call them. Most Mac people buy their new toys and apps from mail order houses or online stores. You have to wait a couple of days but you save tax and they are usually fast and will discount if you bug them. For $3000 you can get a G5 1.8ghz (See MadMaxBR5's Post # 9 above for features) and still have enough to get a LOT of ram and a new LCD panel. Or use one(or two) of your present monitors and you can get some new apps to play with also. Get a bunch of Ram...with Macs, more Ram = more speed and more open large apps. You can get very good prices on Ram from www.datamem.com(for PCs also). Much better prices than from Apple(or Dell). For Factory warranty service you can take Macs to any CompUSA if you have one nearby.
I suggested the 1.8ghz G5 because neither Poser or Bryce are multiprocessor aware and that machine has 8 dimm slots(8 GB Ram Max). You can also upgrade the processor down the road if you want to. If you have any portable drives (USB1or2.0/Firewire IE1394) if you format them for PC's you can use them on all of your machines. Macs will mount all PC media just fine. I use one to transfer large files between my Macs and PCs because it is faster than 100b/T ethernet. The G5 has gigabit ethernet btw. It also has the hardware and software to edit movies and create custom DVDs included. Along with speech control and handwritting recognition(this is great with a Wacom tablet). OSX will also allow file and printer sharing with your PCs so you won't need to move your printer around unless you want to.
You can expect to get SEVERAL years of strong service from a machine like this. My brother is still using one of my 8 years old machines (a UMAX clone) and it still runs ALL current apps. He did upgrade the CPU and put a gig of Ram in it. He won't part with it because it has 6 PCI slots (with most filled.) My old Mac Plus from 1985 still works just fine also.

If you need more info you can IM me if you like.

Welcome
Bill