Bumhug, which 4890s do you have in your UD5 at the moment?
I have the same mobo, and a couple of XFX 4890s on order to replace the OcUK Value ones just bought which were too wide to use in the 16x16x slots.
Was hoping to use one of my old 8800GTs for physx but this doesn't sound good...seems like I should have just gone for 2* 260 SLI, or even my original choice which was a P55 mobo, i-7 860 with a single 5850
LOL, doesn't matter how much research I do I always feel I've made the wrong choice...and in the end I'll just feel I didn't even need to upgrade anyway
I
had two powercolor ones in there. Bit of an epic story really, they fitted with stock coolers but was very little room between them. The card in slot one would get hot so I looked at getting some aftermarket coolers. First attempt was a Zalma Vf2000 which was a huge mistake as didnt read up on it properly and the cooler itself takes up two slots. So stuck that on card 2 with one still running stock.
Was going to put up with the noise but when Grid was cheap on steam I bought that and then the problems started. Game would crash after a couple of races, had crashes like it before screen just locks up sound continues for a bit then system locks up and needs rebooting. But previous crashes would be once a week this was every couple of races. Got MSI Afterburner and was monitoring temps on my G19, card two was on full load at 50c card one was full load above 100c once it hit 110c that was it crashed. So bought 2 Zalman GV1000 with the hope that these would work and free up the 3rd slot, these however turned out to be louder than the stock coolers and didnt make any difference running furmark card one would hit 100c in about 10 seconds and would have to stop it before it crashed.
Bearing in mind got an Antec 1200 case, all fans are on full whack and have fitted a side fan as well, case temp idle is between 35c and 40c cpu just below 40c, when cards are at full load the temp rises to 48c in the case which isnt that hot at all (and considering one card is running over 100c fairly impressive)
So had to stick the VF2000 back on the second card and the stock cooler on card one, but having done this just cant get system to work with 2 cards. Running furmark the system crashes after a few seconds, Grid gets to the menu screen and crashes. Thing is from the look of Afterburner its card 2 thats crashing, not heat related either put some load on the card and it just dies. However if I take out card one and just use the second card it works fine so isnt a problem with the card. The other problem is the fan on card one now makes contact with the screws on the back of card two, which I cant work out as had this setup before and it was fine, have even replaced the screws with slightly smaller ones that should be ok for crossfire
So morale of this story is if the new cards you get arent much better dont waste money on aftermarket cooling as they wont do the job either, have spent £120 trying to get temps down but had no luck at all and am now using 1 less card than when I started (on the plus side it is very quite) but still got £230 worth of kit sitting outside my PC that should be inside.
However saying that the slot between 2 and 3 is wider than one or two so adding a third card wont be cause as many problems, for me though my case runs out of slots there so only have one case slot for a two slot card.
I am probably going to end up selling both cards and getting a 5850 (or a 5870 if prices become sane) with this motherboard doubt I will be going crossfire again as its just been a headache
, and ive been called a troll, been called a liar etc..., for stating that you do get a drop with PhsyX, quick Forn., better get that removed and corrected, before they all steam in calling you a troll, a liar, and claim you've just shopped that. 
