It sorta depends on your setup but Eheim pumps have always been good and reliable. I believe Eheim has the only pumps to offer a molex power connection. That is really handy for completely internal systems.
Well my new system will be cooling the next gen graphics cards (prolly a Geforce 6800GT (i think)), and an AMD Athlon 64. It wont be complete until 2005 sometime. But im looking 2 increase my water cooling knoledge before that time.
Ill be wanting a good relaible pump / near silent / but with a good flow rate. I saw a review of a water cooling system in which the pump made the water do 2-4 cycle of the system everey SECOND!
Flow rate is good, yes. But to much flow rate can have negative effects. If the water is moving to fast it won't be as efficent in taking heat from the waterblock and releasing it through the radiator.
I have almost 1gallon of water in my system and a 185 gallon per hour (about 3gallons per min) pump. It seems to be a pretty good water volume to flow ratio because I get VERY good cooling results (16-20C). The pumps made for completely internal systems are much lower gph but there is much less water to move.
For pumps made for computer cooling Eheims and Danger Den pumps are some of the best. They are quiet, powerful, and reliable.
The only ones I can think of in that price range is the hydor L20 or the eheim 1048. Both have similar flow rates but i think the eheim looks like it is better build quality. The hydor pumps can also be a little noisy if they are not well insulated.
Higher flow rates is the only difference as far as Im aware. If you only wanna spend £50 I would just get a hydor L30, it has a good flow rate and if you stick it in a icecream tub full of water you wont hear it.
Just out of interest did the weather turn [-peep-] in your part of England today aswell?
Im using an asetek KT12 L30 water cooling setup, this cools the CPU, GPU and northbridge of my system aswell as going through a 2x120mm radiator. The hydor L30 pump doesnt have any problem with this so it should be fine for your setup.
To be honest the ehiem pump is prob a little better, but i havent got any experience with it...bare in mind it is also a lot bigger than the hydor one.
My personal preference is a Danner Mag Drive 3. it's quiet (less than 20 Db I'd say since my fans that are 21 Db are slightly louder than it) I definately recommend padding it with rubber since it viberates.
It does put out great flow though 350 gallons per hour to be exact. you could also go with a model down or up depending on your flow need. But they are pretty inexpensive, seem to be reliable as far as I can tell (heck there is only one moving part and that's a magnet that has a impeller attached and you can buy replacements if you do something like poke it with a screwdriver or something else stupid)
Look up some reviews of them. Been a while since I did but that seemed to be the best balance between performance, silence, cost, etc.