Provided you have enough memory I agree with D3matt we ran our backup vps with 25 players with just 0.7ghz x4 allocated for a month with 3 gig allocated to the tekkit server map I think disk access was starting to become an issue but a lot of people commented on how good the server was running with no lag.
I know that a lot of australian tekkit servers are not running on any where near the hardware that they claim either as I chatted with the admin from one so called Aussie tekkit server on my website I use quick chat with quick flag enabled as a chat room, the owner was flagged in the uk and the admin was in the philippines and he quotes the server specifications of the host server not the specifications of his vps.
So don't judge a tekkit server by it quoted specifications or what it plays like either we deliberately ran three tekkit servers into lagg and our quad core xeon was only at 4% load with no ill effects on a fourth server this was done in some early testing that was carried out to better understand the relationship between cpu, java, memory and tekkit.
I think a good tekkit server is 80-90 percent good "server guy admin" tekkit is not minecraft a tekkit server can go all wrong within seconds of an idiot getting his hands on something that he shouldn't I seen it happen when we shut our vps down we enabled creative mode to place signs telling people that we moved and within 20 seconds some idiot run the server into lag
I'm kicking another server in the guts soon I'm tekkit server name challenged I wanted an aussie name so I came up with spinifex :/ @ www.spnfx.com.au it's not up as yet no big rush I'm going to hone our signup prosess a little in an efort to eleviate the idiot player factor as much as possible still chasing that perfict sever I guess