The answer will depend on what exactly you want to do with your PC. Do you want to play games? Make movies? Burn movies to DVD? Edit photographs? Record music? Or just surf the web?
If you want to play games (which requires the most processing power of all) the latest processor will help. But more importantly, you'll want the latest 3D graphics card. That means a dedicated graphics card that is separate from your motherboard. One that has it's own memory and does not "borrow" from your regular RAM memory. In orther words... DO NOT BUY A COMPUTER WITH A BUILT IN GRAPHICS CARD! (Basically avoid anything that says GMA Graphics.)
If you're into making movies, or recording music, or messing with photoshop, the 3D graphics card is less important, since this kind of stuff uses mainly 2D graphics and the quality of your monitor will be more important.
If all you want to do is surf the web and watch movies (either flash movies through your browser, or downloaded movies from your hard drive, or DVD movies through your player) than all you need to worry about is how much RAM memory you have, as any NEW computer will be able to handle what the web has to offer.
So since I don't know what you want to use your PC for exactly, here's a list of things to look for...
1.) The fastest processor on the market (currently Apple's 3.0GHz Dual-Core Intel Xeon "Woodcrest" model. Mainly because the "Woodcrest" Xeon processors that come in the Apple Quad models are actually SERVER processors. More powerful than the regular Xeon processors you'll find in most Windows based PCs. So when looking for a processor these are the words to look for "Two Dual-Core Intel Xeon “Woodcrest” processors at 3GHz each." The "two dual-core" making it a Quad. And the "Woodcrest" being the more powerful Server processors. And of course the 3GHz speed.
2.) The fastest 3D graphics card on the market (If money is truly no object, the Nvidia Quadro FX 4500 is the best. But it does cost between 1500-1700 dollars. If that's too much just for a 3D Graphics card, the Nvidia GeForce 7300 GT card will run you somewhere between 400-500 dollars. Or the Radeon® X1900 series which will cost you between 200-300 dollars.) Again, if gaming is not your thing, then no need to go crazy.
3.) Hard Drive. Make sure your hard drive is an SATA (as opposed to a regular ATA). Make sure it's 7200 RPMs (as opposed to 5400 RPMs). And since internal hard drives are cheap, go as big as you can (you can get a 700GB drive for something between 200-300 dollars)
4.) RAM Memory. You can never have enough RAM memory. Unfortunately, this is the one component of a computer that the corporations have managed to price-fix. So no matter where you look, you will be paying too much for your RAM. I suggest you buy your computer with the lowest amount (as the PC sellers will overcharge you even more) and order the bulk of your RAM from a reputable online store like memoryx.net. I myself have 2.5GBs of RAM in my computer. It's a healthy amount, although I wouldn't mind having more.
5.) A good DVD player. Make sure the following DVD formats are not only readable, but writable with your burner...
CD
DVD+R
DVD-R
DVD+R DL(Double Layer)
DVD-R DL(Dual Layer)
Who knows when the Blu-Ray or DVDHD burners will be available, but I wouldn't worry about that now since no one knows what the new format is going to be yet.
6.) High speed cable internet. Don't be fooled by those DSL ads selling for 19.95 a month. Unless your house is right next to a telephone signal station, you'll be getting around 300kbps, as opposed to 1.5mbps with Cable. But, if money is not an object, then go with DSL and order a T2 or T3 line that is super crazy fast. Otherwise, go with your cable company.
7.) Most importantly, do not buy a new PC running Windows Vista. Vista is a seriously bloated operating system that requires so much RAM and processing power, just to run normally, you won't have much left over to run your programs. It'll be at least a couple of years before Vista is practical to use. By then computers will be faster and stronger. Not to mention the fact that Vista does not yet support all Windows software or hardware yet. It's a big headache and you will be wasting all that super-processing power and RAM just keeping the system running.
8.) If you feel you have to go with a Windows machine, buy one that comes with the full version of Windows XP. Not one of the cheaper versions, but the Professional Version.
9.) If you just want pure power, and top of the line hardware, whether for playing games on XP, or making movies, or recording music, or Photoshopping pictures, your best bet is the Apple Quad. You'll have to install your own copy of Windows XP, but it contains the fastest Intel Processors out there. If you decide to build your own Windows PC, component by component, to match the Apple Quad's Xeon Woodcrest model, you will spend more money than if you just bought the Apple Quad.
Hope this helps!