Yep, mental - 6A for 4 hours would suck a 17AHr battery dry! How long do you want to draw that current for? If say 6-8 hours, you should probably look for a 50-80 AHr battery to give you some reserve by end of the night.
If $s can stretch, then an AGM (Absorbed glass mat) seem to be considered the best.
I bought an 80Ah (I think it was) from these guys, from their ebay shop:- ausjacko
http://stores.ebay.com.au/indyindust...eboardproducts
Fair price - going well so far, but I'll give it a solid workout this coming weekend.