Very low spec will be fine for PXE Reps under many circumstances as I've had these running on very low specced virtual machines or spare old laptops before now. Thing to watch for is network card speed and disk speed especially if you expect to also use them to host images. If planning to use WinPE expect this to be a high disk and network load since that is very large and a low spec system will quickly become a bottleneck if you are trying to PXE boot a number of systems simultaneously. If not then pretty much any system with workstation or server OS is going to be able to handle this.
Mark Star - MarXtar LANDesk Enhancements
Home of Power State Notifier & Wake-On-WAN for LANDesk