For a lot of games 2-key-rollover is enough. But sometimes it won't work for FPS if you need to move forward and right and also reload and jump at the same time. You would need 6KRO or more. Most USB keybaords do 6KRO as that's the default USB implementation for keyboards (6 individual keys plus modifiers: shifts, ctrl, alt, etc). Some keyboards like Leopold can do N-key roll over on USB. Poker can do NKRO as well. Some of the NKRO implementation don't work on OS X though. Most of the 6KRO can become NKRO over PS/2 port, if PS/2 is supported.
Rosewill, CM Storm are value gaming keyboards. CM Storm tenkeyless is around $70 when discounted. Leopold is good at around $110 from elitekeyboards.com. Filco is also very good, at round $140+ from keyboardco fulfilled by Amazon in the USA. Both Leopold and Filco are better made keyboards with better PCB design and manufacturing than Poker, Rosewill, and CM Storm. But you don't game better on the cheaper 6KRO keyboards.
As for linear (Cherry MX red and black) vs. tactile (Cherry MX brown and clear) vs. clicky (Cherry MX blue), it is just a personal preference. There are professional Starcraft 2 players using any of the above. Clicky switches like IBM buckling springs and Cherry MX blue are not as good for gaming if you need to double tap a lot (as in Starcraft 2) because the actuation point and the reset point are far away relative to linear (red/black) and tactile (Cherry MX brown). So when double-tapping, the key switch stem needs to travel a longer distance comparing to linear switches which you could "float" between the actuation and reset points.
I am a noob here on head-fi, but I frequent geekhack.org and deskthority.net with 40+ keyboards in my collection. So I can help out here or over on those 2 forums.
I personally love brown and red, as well as Realforce topre switches and sometimes IBM Model Ms. But just today I am typing on a DAS2 with MX blue and blank keycaps at work. I guess I love them all. :-)
As you might have suspected by my mechanical keyboard talk, I'm a fellow Geekhacker. I do have to admit that I don't browse those forums as much as I used to now that I pretty much know what I want...either a Cherry MX Clear board or a converted IBM Model F 122-key.
In the meantime, in addition to the Model M, I've also got a DSI Modular Mac (6KRO tenkeyless Cherry MX Blue board), two BTC-5339s (foam-and-foil capacitive NKRO boards), a Wang board of some sort (clicky yellow/orange Omrons with ALPS-esque stems), and a Dell AT101W (black ALPS). I cycle between them every now and then, but it's usually all about the Model M and the DSIMM.
Not to mechanical keyboard fans; it's rubber-dome, like all Logitech boards.
If only I could find a way to refit this Logitech G15v1 I found at a thrift store with mechanical keyswitches throughout...it would take an immense amount of work, though, and all I'd really get out of it would be the LCD.