Need help overclocking Sapphire HD 4870

Greetings,

I'm giving consideration to purchasing either an EVGA GeForce GTS 250 with 512 MB 256-bit GDDR3 Ram or an EVGA GeForce 9800 GT 1GB 256-bit GDDR3 Ram. I'm not a gamer but I do a lot of graphic design work so either card should be OK for my needs. Whichever card I purchase to run my ViewSonic VP2130b Pro monitor, I want to replace the on-board cooling fan. I've done my best to make a super quite system so I don't want to ruin my quiet with a screeching video fan.

I have two questions:

1. Which card do you think is the better card for a graphics design application, and
2. Does anyone have a recommendation as to what 'silent' aftermarket fan I could purchase for either of these cards? (This may be the deciding factor for the card.)

I now have a Zalman VF700-CU fan on an EVGA NVidia GeForce 7900GS KO that runs at a silent 800 rpm. It's a great fan that keeps my card a steady temperatures. However, it's not spec'ed to fit either card I want to purchase.

Thanks in advance for your help.

there is an overclocking panel in CCC...use that to overclock and to test. If you start to get flickering (most common) or artifacts then back off. Although I read a review that compared the factory overclocks to non-overclocked reference cards and there wasn't much difference.