Maybe a 750? I run a 1650 and 1250 in my 90 and that is PLENTY.. It's actually ripped up the plants out of the sand before.. I have one deadspot in my tank right by the intake, the good thing is i have the circulation correct in that all the stuff settles by the intake of the canister. The problem is, the canister intake tube isnt long enough to get close enough to the sand to suck it up.. Hopefully my Eheim universal Kit will solve that problem. If it doesn't it makes cleaning easy because you just need to suction from one spot..
I've also found that place the pumps so they hit the front of the glass works best as it spreads out the flow when it hits to the glass to the left and right and will push things towards the back.. This is why I added a second pump to bring it to one corner..
That is what I would try to achieve.