Hi! Ahhh- fun time
Since you are comfy on a PC already, stick there vs Mac.
I would recommend staying away from older chipsets (not too hard), and SiS chipsets. 845e or higher, or 850 work great. 845's use DDR ram, while 850's use RDRAM. RDRAM has a slight benefit over DDR, even the new dual-pipe DDR boards. I would also suggest going with a board that supports the new Intel hyper-threading, allowing upgrading later. I would also recommend going with a 2.53G CPU or higher; they are using a higher FSB naively. ASUS is my preferred for a board, but there are other good choices- stay major name here IMO. Onboard USB (make sure it has USB2 as well). I prefer onboard LAN 10/100 as well, save s a PCI slot. Some even have onboard Firewire, saving yet another slot. With RAM, try to use the best quality, 512 is great, unless you intend to run virtual instruments or other that cache to ram- if so, add more ram.
For video, I prefer an LCD for music- no interference ala CRT's, especially if self-tracking guitar or bass
For a card, there are many good choices. Matrox makes good ones, although I prefer Nvidia (Geforce4 Ti series almost all allow dual monitors). Avoid onboard video!
OS. Win XP Pro- since you will be buying a hard drive (or two), $140-150 will get you an OEM Win XP Pro. Then hit tweakxp.com or whatever and a few other sites for music specific tweaks, and it should be rock stable.
Sound card. Fun. Onboard audio is all over the place. Some use a codec that caches to ram for the "synth" part, whether you use it or not. SB Live OEM are cheap, and great as a second out if needed. For primary I/O for recording, RME would be my first choice, although there are other great ones out there. Keeping the convertors and such outside the computer is suggested.
Hard drives. WD is great, IMO. The drive you chose for primary drive is great, but I would grab a WD 120G 7200 8M cache special edition drive for the secondary (dedicated to audio tracks); only a bit more, and worth it, IMO.
CD-DVD. There are more combo drives emerging. If you can afford it, I would grab a Pioneer A05 DVD writer. Recording at 44.1/24bit, files pile up quickly. Avoid the DVD+R/RW drives though. The Pioneer does CD-R/RW and DVD-R/RW, and discs burned in it will play back (or restore) on most DVD-ROM drives, and DVD video/audio burned will play on most consumer DVD stand-alone units; not so with DVD+R/RW. There are DVD-ROM/CD-R/RW (plays DVD's, records/plays CD-R/RW) units out there now for about $150+ (newegg.com- shop a bit).
Case and power supply. I prefer a full tower, more breathing room, easier to work in when needed. Love the side-windowed cases- can see whats going on (fans and such). Aluminum/plastic cases are lighter, but I feel the metal cases are a little quieter. I put some Akasha Mat sound proofing in as well. For a power supply, I would suggest a 400watt+ Antec or Enermax with quiet fans. I LOVE the Antec True Blue- quiet, accurate power rating, dependable. At newegg, I like the Chieftec side-window cases; the access side comes off with a simple push of a latch- much like the new Mac G4's. Easiest access yet.
Sonar would be my first suggestion since you are already familiar with Cakewalk. Sonar has come a long way, and support of the DXi format is booming as well.
Any other questions, pls ask- happy to help if I can. I have built 2 music computers, and one gaming computer in the past 6 months for myself, and did alot of research before spending any money. I use an MBox on my gaming computer for scratch ideas, Pro Tools HD3 on one music computer, and Cubase SX and Reason (and a few other virtual instruments) on the other. Hope some of this helps. nikki