You got a pc? Battlefield 2013 (or somesuch) is supposed to be a good game. Haven't played it myself though.
Though the best Sci-Fi shooters for PC are the games in the STALKER series.
For ps2 I could recommend... uhm... well, i've had a ps2 for, well, forever, and I cant think of anything else than Killzone.
Killzone is pretty violent, very good graphic (by ps2 standards anyway) and HARD, but is very well made, and also very much SciFi.