If cost really isn't an object you could try a Mac Pro (same as I've got - or with even more options) It can be set to boot into Windows - but you have to buy Windows seperately and download Apple's Boot Camp. There is a Mac version of Office, but it hasn't been converted to run natively on Intel processors yet.
Memory is a bit pricey (FB RAM with extra heat sinks to keep the temperature down).
Otherwise, with options it meets or exceeds all your specs.
TWO Core 2 Duo Xeon processors (4 cores) (2.66 GHz standard - 3.0 GHz or 2.0 GHz as options)
1 GB RAM (Plenty of expansion room - there are 4 pairs of slots. Installing your own memory isn't difficult and may be cheaper - but try to get good quality memory)
250 GB HD (Bigger drives as options - I have 500 GB - and installing addditional drives is incredibly simple - there are 4 3.5" bays for SATA drives)
nVidia 7300 graphics card with 256 MB Ram (ATi X1900 available as an option - and probably more than you need) The standard card can run a 30" monitor
DVD/CD writer (Apple Superdrive)
Apple isn't perfect but it still has a better reputation than most.
MacPro - Apple
And it runs OS X :-)
™ Version 4.2
