If you have more than 3.5 gb of ram, go with 64 bit - its almost certainly a 64 bit system.
A more scientific way to find out is to look up if your processor supports EMT64T - you can look this up on intel's website or use a suitable tool to find out - on windows you can use cpu-z
This is a 64 bit system and has the EMT64T instruction (AMD systems may have a AMD64 instruction)
If these are systems with no os, boot a linux livecd and use /proc/cpuinfo - you're looking for the lm flag under flags so grep -E lm /proc/cpuinfo should have no output on a 32 bit system, and spit out everything under 'flags' including an entry that says lm
The one on my VPS looks like this, and they're running a 64 bit xeon core.
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ss ht syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon rep_good pni pclmulqdq ssse3 cx16 sse4_1 sse4_2 popcnt aes lahf_lm If you find that either tool tells you its 64 bit capable chances are you can run a 64 bit os, assuming you meet the other system requirements. Otherwise you will have to go for 32 bit.