I just built a new PC with OS X and Logic in mind. I have also ran OS X and Logic on my old Dell Dimension 4400 desktop ( gotta love those free company disposals that come right when your PC dies

) and my Dell Inspiron 5100.
The system I built myself runs OS X so much more reliably then either my my Dell systems (both SSE2 CPU's) although its not all that bad on my 5100. I suppose if you bought a system that checks out 100% OK on the HCL then it'd be fine...but I strongly recommend just building the PC yourself. My setup cost just under $800 and its more powerful then any pre-built PC you can find at the price.
Intel Core 2 Duo E6300 CPU
Gigabyte 965P-DS3 Motherboard (1333 FSB

)
1GB DDR2-800 RAM
160GB WD SATA2 HDD
GeForce 7300 LE 128MB Video Card
AeroCool ExtremEngine 3T Case
Liteon SATA DVD-+RW
400W High performance PSU
I was going to pick up an M-Audio Delta series audio card but after reading about the issues with PCI I decided to just use the onboard for the time being until the PCI Express cards get released and have stable drivers. I'd go firewire except that I really don't want another thing cluttering up my desk and there's no native support for firewire on the DS3.
Whatever you do make sure the system you get has a SSE3 CPU. While you can run OS X and Logic on an SSE2 it can get pretty slow at times which gets frustrating after a while.