Upgrading my computer for X2, what's a good choice under $1000?

I've just checked the specs for Sonar X2 and realized that I will be unable to use my trusty Athlon X2 4800+ machine running Win XP SP3.  Too bad, because that machine has plenty of grunt for the projects I do.  I'll keep it around for a backup.

Anyway, I plan to spend $1000 or less, which should be plenty for a potent machine.  Can someone please advise what I should look for, or assemble, for my next computer?  Also please advise on whether I should plan on installing Win 7 32bit or 64 bit, and what to do about all my 32-bit plugins if the answer is 64-bit.

The specs for X2 say that it needs a quad core processor, but if you think my Athlon X2 4800+ machine will run Win 7 fast enough for Sonar, please say so.

Thanks in advance, LR

My new PC specs are listed in the referenced thread.  I built my PC two months ago. 

Go with 64 bit OS as it will address more than 4GB of RAM (and you should install a minimum of 8 GB and even 16GB if you use a lot of MIDI and samples).  A 32 bit OS will not fully use 4GB of RAM and after system use you wlll only have 3 to 3.5GB of useable memory.