What's the temp outside the cabinet? Inside? What's the cfm rating of the fans? Are you using any kind of filters over them, because this will drastically reduce their efficacy. Will you be using them both as exhaust with passive intake? A little more information is necessary.
ok ambient temps outside cab will be 22-23°C and inside i was thinking 29°C. i was thinking of using a 250w hps first but those get very hot. no filters but some simple vented hood (airtight) with the two fans.http://www.nexustek.nl/120mmcasefan.htm
Kinda weak for 120mm fans, theyre usually rated at 90 cfm. I think you should aim above 100cfm, so another one or two of those fans might do it.
Remember that fans do absolutley nothing if you place them in the wrong place. Intakes/passive intakes should be near the bottom of the cab, and the exhaust should be on the top near the light.