Depends on a number of things Dude. If you have experience that is in high demand with the employer then I wouldn't worry too much. A suit (like any other individual thing) is not really going to make or break it (unless it really is extreme!), but it is all about your overall impression. Perhaps your suit is inappropriate, you don't answer a few questions very well, your hair is a little untidy, your sim performance marginal etc etc and the overall impression is not great, possibly leading to someone else being offered the position.
If you can afford a new suit then it is one more element of the selection process that you are able to control and I feel therefore you should control. There are many other elements (which should be regarded as more important) that can also be controlled and to give yourself the best chance of being in the melting pot once the selections are complete and they are handing out the jobs, then I believe that you must control everything that you possibly can. It could make the difference between an 'ok' performance and a good one. You want to be one of the 'definites' rather than the 'possible/ maybes' or the 'thanks, but no thanks' groups.
PP