We are a small group like yourselves, 2-4 players. We started on gold (Robert's recommendation based on mod pack & number of players) with the view that with no migration costs from package to package we could move up or down if necessary. We are still messing about with 1.5.2ngt pack so I wouldn't say it's been 100% committed play from all of us, but we've made an effort to push what we could in terms of testing the server performance (we plan on switching packs & world soon), the 1gb memory has never been an issue & tps has sat happily at the 20 mark (used benchillTPS & servertools to read) unless I forget the wrong (inefficient) mob grinders running. We are running multiple mystcraft worlds, high end bees with multiple alviaries, 24/7 slime embigenner grinder, generally 1 other 24/7 grinder produce by either T5 shards or MFR spawners - whether its zombie pigman or spiders or enders (depends), 16 HP boilers, full ore processing lines, several MFR farms & 11 forestry farms, 20 turtle mining rig, 20 unit recycler run & massfab, 500unit/t AE system, plus whatever else is kicking about in there - not massive but I think you can get a baseline? To be honest I have looked around at some of the other pricing & for instance Creeper host would work out to a few bucks less for us, so something to consider if you budget is to be considered?
edit: Gold = 1024mb @ $15p/m. They are using AMD server hardware, not sure if this is for every pack though you would need to check if that's a prob for you & although I am not sure I reckon it must be virtual for that price. Also no wait on server setup its done pretty much instantly.