If you really want to be safe, stick with hardware on PTC's compatibility list. You'll find that in the support.ptc.com area. It usually includes the big players like Dell, HP, IBM/Lenovo. I've noticed that they tend to support Windows the most (Solaris is the only other OS supported in the WF5). Additionally, they tend to stick to only ATI and nVidia for graphics hardware. This doesn't mean that other systems won't work, but these are certified and supported by PTC so you should "less" issues.
If you're running large assemblies get a lot of RAM, but remember that Windows 32-bit only supports ~4GB total system memory, so you'll need to run a 64-bit OS if you want more than 4GB.
We've been running Dells for quite some time. The current Dell M4400 seem to be a good deal faster (using SPEC benchmark) than the Dell M65/M4300 series.
SurfaceInk Corporation - Boston