Money not an issue....Let's see. This guide (
http://www.anandtech.com/guides/showdoc.aspx?i=2753) from anandtech attempts to build the best system possible for under $1500, which is a good idea for your first system. You want the best out there, you'll be wanting to look for an AMD Athlon 64 FX-60 processor or an Intel Extreme Edition Dual Core processor (either one costing ~$800-$1000, with AMD being a bit cheaper and just as good if not better), a good motherboard (MSI is a great company, and I understand their MSI K8N Diamond Plus is a good board for the AMD chip), 2-4 GB of good memory (I would recommend Kingston Tech), a Sound Blaster X-Fi soundcard (although a Soundblaster Audigy 2 card would do nicely as well), an ATI Radeon X1900XTX or (even better as it is faster) a dual-processor GeForce 7950GX2 card (or two of them in SLI, which means they share the video processing and increase the computer's speed, when the drivers come out). The speakers and keyboard/mouse mentioned in the above guide are supposed to be good, although I personally am a fan of the Logitech X700 or X1000 cordless mouse, and type of keyboard is really a thing of personal preference. The article also mentions good choices for a DVD/CD burners and hard drives, and if you want a larger hard drive simply buy a bigger model from Maxtor, Western Digital, or Seagate. I would estimate the no-holds-barred machine as costing upwards of $3000 easily, and very likely more like $4000, but the lower end machine would play games really well, and the extra cost isn't always worth it. As for the best gaming OS, right now it would be Windows XP, no contest. No other OS has the number or quality of games that Windows XP does. You may pick other OS's for other reasons, but for gaming it has to be Windows XP (although Windows Vista might be better after it comes out and gains support).
"...cease not a day from this work; be killing sin or it will be killing you." - John Owen The Mortification of Sin