Gaming keyboards are often advertised with an "anti-ghosting" feature, and its there to advertise the ability to press multiple keys, and while it has to do with what they're advertising, its not technically correct.
The correct term is
key rollover, ghosting is what happens when a key actuation occurs for a key that wasn't actually pressed, which can occur when multiple key presses are attempted and fails. The problem with this is a keyboard can be 2KRO and not ghost, while others can be NKRO and have ghosting problems (Microsoft's Sidewinder X4 is an example NKRO + ghosting problem)
Anways, a keyboard that fails when more than any two keys fail a pressed and the 3rd+ key(s) fail to actuate is called a 2KRO board, which is what I referred to just previously.
IIRC, USB is capable of up to 6KRO (plus the shift/ctrl/alt keys which can make it up to 10KRO), however PS/2 is capable of
n-key rollover, ie
NKRO, which means any number of keys can be pressed and all of them will be recognized.
if you want to really look into it more, they guys at geekhack.org are nuts about keyboards and pick them apart thoroughly and mercilessly:
http://geekhack.org/showwiki.php?title=NKey+Rollover+-+Overview+Testing+Methodology+and+Results
@ $100 new, SteelSeries 6Gv2 will probably be (on average) the least expensive and easiest to find keyboard capable of NKRO over PS/2 or 6KRO over USB. It uses Cherry MX Black mechanical switches which I've seen quite a few people have an either love it or hate it stance with. I have the 6Gv2 and absolutely love it. The biggest complaint about the Cherry MX Black switch is that it requires the most force out of all the Cherry MX switches (60g to actuation) whereas other keys like the Browns and Reds only require 45g to actuation, or the Blues and Clears 50g and 55g respectively.
All Cherry MX switches actuate at 2mm into the keystroke and bottom out at 4mm. Browns, Clears and Blues have a tactile bump added to them to help give users physical feed back that their key press has registered and actuated, the Blues go a step further and add a distinctly audible click at the actuation distance along with the tactile bump.
Blacks and Reds are linear switches, meaning there will not be any tactile bump at the actuation point. This is generally considered ideal for certain movement oriented game types because you can literally hover the key right at the actuation point and spam away individual key presses with the greatest speed and precision, with emphasis on speed.
The positive about the tactile switches are the fact that they give feedback about key presses, so games that might require distinct and accurate key presses might benefit more from such switches (casting spells, issuing shortcut commands, or rhythm/pattern matching)
That being said, here are some options (all will be NKRO over PS/2 or 6KRO over USB):
$129-135 -
http://www.daskeyboard.com/ - provides Cherry MX Blue and Brown options
$132-143 -
http://www.elitekeyboards.com/products.php?sub=filco_keyboards - the NKRO Filco boards (there are both 104 and 87 key variants) can be found in Blue, Brown, and Black, although the Browns are generally the most poplar and are thus hardly ever in stock.
$119-159 -
http://www.deckkeyboards.com/keyswitches.php - they have Black and Clear options. And also an 82 key option for $119. Some of the most expensive I'm going to recommend but still within your budget. They include back lit keys and have a pretty good 30 day money back guarantee.
$99-149 -
http://steelseries.com/us/products/keyboards - the 6Gv2 and 7G are both Cherry Black switches and nearly identical. The $50 extra for the 7G gets you USB/audio passthrough ports and a removable hand rest.
$130 -
http://www.xarmor-usa.com/index.php?dispatch=products.view&product_id=1 - The XArmor U9BL has Cherry Blue switches and blue back lighting.