With $1500 you can get one of the most powerful PC with Quadruple core processor and RAM to spare, same identical hardware on the Mac would cost you around $3500. I recommend you get the 64 bit version of the Operating System because DDR2 RAM is very cheap nowadays, you can get 4GB for just around $80 (DD3 is faster and more expensive being the "new" memory but prices are dropping quickly as more people chose it); ONLY a 64 bit OS can address more than 3GB of RAM, so if you have, say, Vista 32 bit is totally useless to put 4GB as it will see it but won't be able to use it. With that your PC will be very responsive and capable to handle large image files without delays due to swapping from the Hard Drive.
This is my recommended setup:
AMD Phenom II X4 925 Quad-Core CPU (The Intel i7 CPU is a bit faster, but also much more expensive and uses more power)
8GB of DDR3 RAM (4 X 2GB bars)
NVIDIA GeForce 9500 GT 1GB Graphic Card (the difference in price between the 512MB and 1GB is minimal, just $20, might as well get more memory to free the CPU)
RAID0 1TB Hard Drive setup (this means 2 Hard drives of 500GB each used as if they were one, when you save files each drive will save half almost doubling the effective speed of the drives, same when reading that file. Makes a huge difference with large RAW files and loading large software).
1TB extra hard drive (3rd hard drive) for backup of your images (better safe than sorry). Cheaper and faster than an external drive.
(I get about 80MB/sec transfer with mine).
Windows Vista Business 64 bit with free upgrade to Vista 7 Professional 64 bit
All this is going to cost you about $1050 and you'll have a speedy computer that will be good for years (setup includes keyboard, mouse, wi-fi card). Since this is below your budget I recommend adding a nice 24" High Definition screen (another $400 or so).
Hope this helps.
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