Choosing a mtn bike, suspension options?

Hey all...

Looking into buying a mtn bike, because I'm considering an off-road tri as my first venture into triathlons next spring (one of the Xterra events).  I have mtn biked just a few times casually in the past, but I really qualify as a rank amateur at best.

I have about a million questions, but for now I'll boil it down to just one:  Full suspension, or not? 

Obviously for casual trail riding and/or downhill racing the full suspension bikes are quite tempting, but what about for triathlon racing purposes?  I do understand the issues of improved downhill performance and greater overall comfort on a full susp bike, vs. superior acceleration and climbing capability of a hard-tail...   I'm just not sure which is the most appropriate for a race application.

Yes, I'm also aware that some of the newer models offer manual suspension lockouts that you can toggle back and forth depending on whether you are climbing or descending - awesome concept, but for now seemingly out of my price range.  I really want to get into a good quality yet still starter-budget bike - it still remains to be seen if I'm in this for the long haul.  It seems that finding a gently used mid-range bike may be the most cost-effective option (i.e. the barely used 2004 Giant VT1 that I have found for $900 locally - retailed for over $3K new).

So, input?  Please?

Chris