$8,000 goes fast though, Shooter has NO Mac software, or hardware.
First of all, if I were you, I wouldn't go all out. Every Mac fan is waiting for the release of the G5, we don't know when it will be, but we all want it. So if this is going to be your last computer for a few years, I'd at least try to save something basically to say "Look mom, I only spent 3K on the computer, now can I get this new G5?"
Judging on what I know about you, you're not going to use many of the PowerMac's features. I doubt you'll ever use a PCI card, or SCSI adapter. It's likely the only thing you'll ever add is a harddrive or new video card. But since you play games (haha, no Half-Life for you!), you might want to buy a PowerMac G4 just so you can have something better than the GeForce 2 MX. GeForce 4 MX, Radeon 8500, or the GeForce 4 Ti in june would suit gaming needs better.
But really, the G5 is coming soon, and pretty much everyone is waiting for it, so try to save some money, or you'll be stuck with what you get for a while. If you follow Macs you know they only release big products at certain times, at MacWorld. The G5 might come as soon as July with MacWorld New York, it might not. But maybe get a decent PowerMac G5, not one that costs $5K, and then put your money elsewhere, like into software, into a Palm, iPod, etc.
Here's my suggestion. Cheap, but will be able to run and play everything perfectly, and will give you some money to set away to a G5 fund, like everyone else.
800mhz PowerMac G4 (that's the tower)
256MB RAM, then buy another 256MB at CompUSA for cheaper
60GB HD
Combo (DVD/CD-RW) - I've got the SuperDrive, only used it once, for data purposes
NVIDIA GeForce 4 MX - Good graphics, not nearly the price of the Ti.
250MB Zip - Easy way to transfer some of your PC files over, unless you want to burn it onto CDs
AppleCare plan
$2,197
Now more RAM from CompUSA will be cheap. Also, ATI has some new Radeon 8500 offer where you send them your old card and save money. You might be able to go with the 7500, and then trade up for cheap. The 8500 is a nice card, and will play games quite nicely.
19" CRT display. Can buy PC ones, much cheaper than Apple's flat-panel.
Buy some speakers. No sound cards for Mac means no surround sound. So just get a 3-piece set for cheap, and get better sound than the cheesy mono thing.
10GB iPod - Buy from CompUSA, like i mentioned before.
Palm m505 - My dream Palm

But hey, it's worth the price.
Microsoft IntelliMouse Explorer - Gotta love it.
Software:
QuickTime Pro license - $30
Norton systemWorks 2.0
StuffIt Deluxe 6.5
Toast 5 Titanium
Return to Castle Wolfenstein
Black and White
Unreal Torunament
Office X
Always need a few games. I wouldn't re-buy too many if you don't play them anymore. Just wait for new ones, like Warcraft 3. Blizzard games are hybrids, so those should work fine (D2 & StarCraft (OS X patch avaliable), and WarCraft II for sure).
All and all, you should be under $4,000, with software and everything. Then have your parents put aside the $4,000 for the next computer you get, like a G5. It'll keep her happy, and you'll want a new computer in a year or two anyway. Plus, when you buy a new one again, you don't have to rebuy software, monitor, etc.