some entry level Core 2 Duos don't support 64-bit.
All Intel & AMD desktop CPU's have been 64 bit capable for well over a year although Intel have flagged behind with thier mobile offerings as the Core Duo range do not have EMT64 only the Core 2 Duo + Celeorn M500 chips do.

2GB of ram is a good option at the moment as its so cheap. Some of the integrated graphics solutions coming through at the moment are very capable if you are going with an AMD CPU then the ATI/AMD 690G chipset actually has the best performance of any onboard graphics currently, for Intel CPUs even thier own GMA 950/3000/3100 can handle Vista fine or if you wanted high performance onboad graphics (really no need if its just for workstations) the new Nvidia 7 series chipsets are looking good.

I know some people like to be able to upgrade thier hardware down the line but this is becoming increasingly more difficult, Intel & AMD will both be on new processor sockets towards the end of Q4 2008 and Q1 2009 and also you will most likley see DDR3 in dominance at this time. Working on this information it might be worth planning for best case (being able to upgrade CPU/Ram etc 1-2 years down the line) and worst case to have the spec you buy last until the machines are next replaced.