I would not get a HELOC now unless it's one where you can lock in the rate. Fed Funds rate will start move upwards again (hard to say how much or how fast) but if your HELOC rate moving upwards gives you a rash, then don't do it. The cap for a HELOC is 18%. (Forgive me if someone all ready addressed this).
Even if your rate remains low (single digits); most people begin to panic once the rate begins to trend upwards.
Also, one factor to consider whether to buy now or in 2008 may be what your loan amount is (I did not see it referenced when I was scanning the responses here). If you're in the temporary loan limits of $417,001-$567,500 for conforming or $362,950-$567,500; you may want to consider taking advantage of "conforming jumbo" or "fha jumbo"...at least check it out so you can make an informed decision.
It's possible that Congress may grant an extension beyond Dec. 31, 2008; but we don't know at this point and at what amount.