I tried to search but I didn't find a definite answer.

So is it a good idea to replace the two exhaust 12cm tricools with 800rpm slipstreams? According to some sources they shouldn't be mounted horizontally but if I do so will they die quickly?

Also how much quieter are the 800rpm slipstreams? Do I need a fan controller?

Please help a newbie.

I like slipstreams although they shoudlnt be considered a long life fan, sleeve bearing fans dont like horizontal placement and according to reviews it might lower the life of the bearing... how much is hard to say, for horizontal placement search for Scythe Gentle Typhoons, they use a real bearing. There are other options of fans that you might want to consider, like Nexus BASIC D12SL-12 120mm Case Fan or if you have a higher budget, Noiseblocker NB-Multiframe M12-S1 120mmx25mm Ultra Silent Fan - 750 RPM - below 6 dBA (in my experience its more like 850rpm).

Hard to say, each setup is different, depends a lot into what other components you will have. When ever i built a PC wihout PWM fans i do recommend to use a fan controller, so in case you dont like or even dont need the 800rpm speed you can lower it to your personal preference/need. But you can add this later if you feel you need it, you can also use cheap single fan controllers like zalman fanmate 2, etc. A fan controller will also allow you to go with higher rpm fan and lower it to your personal preference, and you still can ramp it up if its summer or if you were to do anything that will demand much more cooling.

1. 800rpm slipstreams are pretty quiet. See:
world's quietest fan
viewtopic.php?p=541089#p541089

2. I have had a slipstream fail on me before its time. But I have also had a high end NMB Panaflow fan with real high end bearings fail on me before its time. It is hard to use anecdotal reports to determine whether or not a fan has a high failure rate.

3. If you have a failed fan, you can usually fix it by oiling it. See:
Oiling a Noisy Fan
viewtopic.php?p=541788#p541788

Depends mostly on how much air you need to push to keep things reasonably cool. If you'll be using two Slip Stream 800's as case exhaust then most non-overclocked systems can suffice with having them running really quietly on a mere 5V via some cheap fan connector adapter or resistor. If the rest of the system has relatively quiet parts but you still need more exhaust then it's a good idea to try to find the sub 800 RPM sweetspot. With a system that does not have particularly quiet parts one can often stick a Slip Stream 800 fan in and leave it at standard speed, still making a good difference over what's included with most cases.

Sleeve bearing fans that aren't running much faster than 1000 RPM can be expected to live for years even horisontally in a case fan position. One problem that some sleeved fans have is that they may start making odd noises when horisontal like ticking or so, but I haven't picked up anything like that with the Slip Stream 800 model.