Well there are quite a few options. Personally I'm not a fan of 5150, but I know plenty of others that are, so I'll let them sing it's praises.
Chaos on Cronos - I've got the rulebook, read through, but yet to play it. It looks good, there's a good write up of some battles with it up at the moment by Agger. It uses the very popular goal system, which is vastly different from GW like games, and is very engaging. You could possibily also use some of the skills found in Supersystem to get more craziness in the game.
There's also Combat Zone from EM4, quite k rules, but I found they struggled to create some of the alien nasties and weird scifi tech that I wanted.
Flying Lead, is a great game based on the very popular Song of Blades and Heroes engine, it's quite new, and the few games I've had with it so far have been quite good, if you used some of the abilities from 'Mutants and Death Ray Guns' then you'd be able to field a predator quite well.
Then there's Crimson Dust, which I've written myself (see my signiture at the bottom), as I couldn't find anything that suited me, I decided to write my own, and this is the result, though I'm still twicking it.
Duncan