Reddit reviews Kingston Technology ValueRAM 8GB Kit (2x4GB) DDR3 1333 MHz DIMM Desktop Server Memory KVR1333D3E9SK2/8G

1: You don't need big guns for NN/JT on a small cluster, but I assume you are running something on that box anyway to put it to use.
2: Yes for config, ehh for settings butyouknewthatsorryforpointingitoutlikeyoudidnt.... ._. still, be weary of how multi-threading is handled
3: I didn't, I was an intern for all this - and my DBA mentor hated the whole deal, and long term its not how you wanna do it, especially if what you're doing may or may not be breaking some privacy policies... Some of the cheaper hardware I was looking at rounded in at $6k for 8 nodes with 32 intel cores and 64G ram total. I doubt you need that power, but that's what I was considering to chomp through 5-10G raw data per day on a timely basis, given that up to 100 pig queries with multiple joins were the task at hand.

500G of input with group, join, join, order (about 4 maps / 3 reduces per job) took over 3 hours for 3 nodes w/(4cores..6Gmem..1Tdisk as /tmp and /data). Hosting some of the files on mem and not writing to /tmp really cut down on that replicated joins, but I could only do it in rare circumstances, 6G really isn't a lot @_@