Is your HELOC purchase indebtedness (i.e. do you get to deduct the interest as if it were your mortgage)? What's your tax bracket?
For me, my HELOC just floated to 5.5%, but because I'm in the 33% federal bracket and it's treated like my mortgage, its effective rate is closer to 3.685%. I've split the difference and send about 80% of my "extra" money to a Vanguard fund, and 20% to my HELOC.