Question: I'm moving a couple of servers to a colo and was wondering what you would recommend for a hardware firewall to sit in front of them? Is it fine to just get the cheapest Cisco/Fortigate/Juniper/whatever firewall? I don't need anything fancy, pretty much just port forwarding.

Answer: 
You are probably looking at "wasting" at least 1U of rack space for this firewall.

I would not buy a consumer-grade cheapie firewall.

The Juniper Netscreen SSG5 would probably meet your needs, but it is a paperback size format and doesn't come with rack arms (that I recall). The first "rackable" SSG is the SSG140, but that's not quite so cheap -- definitely overkill for your application here.

If you can figure out a way to mount it neatly, the SSG5 would almost certainly be sufficient.
Alternatively - get a Mikrotik RB1100 and see how far it lasts (50mbit for smallish packets was on the table by someone running game servers).

It is CHEAP and has a TON of features in RouterOS. Uses very little power, too.

Then later you can upgrade to something more powerfull if needed. Again, the RB1100 is CHEAP to start with.