42C load is pretty good in my opinion. I am a amd guy but from what I hear prescotts run hotter then hell especially when overclocked. your system should be able to handle the addition of your gpu with no problem. The sign of a good watercooling step is not only how cool your cpu stays at load but how rapidly it cools down when the load is removed and I would bet your system would cool down very quickly. But hten again what te hell go for the 2 extra fans and let me know how it works out. what is the worst that could happen? it will only cost $20 for the two fans so if you want to expieriment go for it. Here is a link for undervolting a fan that is very informative.
http://www.silentpcreview.com/article6-page1.html
her is a excert from the article you should pay attention to if you have not done this before
Caution! There are people, who will say "You can't feed current back into your power-supply" and they are right!
If you plug something between the 12V and 5V lead, you have to be absolutely sure, that no current is fed back into the PSU. Which means, that you need to have other loads on that 5V lead, which suck out more current than you feed to that lead from the 12V lead.
For example: Your fan uses 100mA at 7V. You take 100mA from the 12V lead and feed them into the 5V lead. If you have a different fan (or whatsoever), that sucks out 200mA from the 5V lead and feeds them to ground, you're fine. 100mA are sucked out of the 5V lead, 100mA come from the 12V lead and 200mA go to ground.
Since usual fans only use 1...3W, you are usually perfectly safe with using the 7V trick there. There a lots of devices in typical computers, which draw much more power out. Peltiers or things with power consumptions running up to dozens of watts, require careful calculation of what goes out of the 5V lead and what's going in there from the 12V lead.