For guys, a suit is a must. Most likely black, charcoal grey or navy blue...and a relatively conservative shirt and tie. Don't stress over it...i read a bunch of stuff on here and other sites that got me all stressed out over whether my suit was too cheap and my shoes were formal enough. Don't worry about it too much, just match and look professional...honestly, I don't think they really care about it too much, your personality and knoweledge is way more important! It was kind of funny though, I walked in and the the first thing the interviewer said was nice suit! He was wearing the exact same one!
Definitely wear a full suit not just shirt and tie w/o jacket. Out of the 4 interviews i went to this cycle only 1 guy wore a shirt and tie without the jacket and man did he stand out (he looked very uncomfortable too so I don't think he meant to !!) as far as the better quality suit I think its worth the extra $$, better fit + less wrinkle prone = more relaxed for me so I can focus on more important things during the interview
Just had an interview last Friday, and everyone was in a black or navy suit. All the guys had on really nice dress shoes, and 95 percent of the girls were wearing pants suits. The entire group was exceptionally professional-looking.