hello out there !!!

i'm new to photography and this site so forgive me if i sound like i don't know what i'm doing or talking about...
i need some advice:

i've got a nikon D50 and i shoot surf photography from the beach (surfers are approx. anywhere from 60-80 yards out, maybe even a football field length)...
anyway, i use a 70-300mm lens with a 2x converter...i'm usually going from 200 to 300 mm (maximum zoom) with the lens, and with the 2x converter it makes it 600 mm...
anyway, my pictures come out soft, not as sharp as i'd like...i'm assuming it's because of the 2x converter and the zoom...

my question is this:
my subjects come out large enough (i would like to even zoom in a little more if i could), but the subject is soft...what kind of lens (mm-wise) would i need to make subjects come out sharper and closer?
Also, after getting the photo into my computer, i'll crop it to make the subject look bigger, but how do i know how big my print can be?? i'm thinking that by cropping, the print will come out grainy if i go big (8-1/2x11 or even 20x30).

i don't have the cash to spend BIG bucks on a spectacular lens (who does unless you work for Sports Illustrated and they buy your lens for you)...
will a 400-500 mm lens work and then i just crop my photo without it getting grainy when enlarged???

your advice/help is greatly appreciated////