Matching hardware on an old server

My chiropractor (also my customer) has an old server.  I believe everything is adequately backed up, all the data, but if some day the server dies I want a second server mirroring the first closely enough that I could simply take the hard drive from the first server, put it in the second, and we’d be back in business again

But the server isn’t off the shelf.  It was put together by another IT guy years ago. Where can I buy another server close enough to the first that if I were to take out the hard drive and put it in Microsoft wouldn’t scream about new hardware or we wouldn’t get a BSOD.  
It doesn't have to match exactly.  It could be a better/newer server.  But I must be able to switch from one to the other seamlessly.  My back depends on it. (LOL)

Following are the specs for the current machine:
AMD Athlon 64 processor 3400+  2.20 ghz  1.93 GB ram
Microsoft Windows Server 2003 R2 32bit standard edition, service pack 2.
RAM 2GB single channel DDR @156mhz
Motherboard K8m800-m2 Socket 754
Grahpics 64MB VIA/SBG UniChrome JGP(elitegroup)
Hard drive:  466GB Seagate ST3500 (sata drive).
Audio Realek AC97 Audio for VIA Audio Controller.
K8m 800 m2(socket 754) chipset vendor: VIA Chipset Model VT8237
Southbridege version 00
BIO Phoenix Technologies LTD
Version6.00 PG Date: 12/20/2005
Realtec RTL8169 gigabyte ethernet.

Thanks,
Al