I'm deciding between the Xtreme Plus II and the TT2 (which is about half the price) and was wondering whether the latter would be sufficient to silently cool an unlocked, overclocked 6950.

I won't be using it for folding, etc only gaming, but of course I will run Furmark and other benchmarks from time to time, so it needs to be able to handle that as well.

According to AC, the TT II can handle up to an absolute maximum of 250W.

A standard 6970 draw around 200W when gaming, and about 260W with FurMark.
An oc'ed 6950 is taxed by ~40W more over a reference one.

I think that you may do your math: IMVHO the TT II shouldn't be able to handle your unlocked and oc'ed 6950 *quietly* (if not at idle).

You might be right.

I recall however that some people used the Xtreme Plus (and Xtreme 5870) on the 6950 (and other cards with similar TDP) and reported it was quiet, and they're only rated to 250W as well.

I'm not sure what the difference between the Plus and Plus II is (the Plus says it has 5 heatpipes and 84 fins and the Plus II says 5 heatpipes and 83 fins) that allows the Plus II to cool an extra 50W.

I also note that the Plus is stated to have a noise Level of 0.5 sone and 81CFM, 900-2000RPM (no noise level is given for the Plus II but the CFM & RPM are the same) whilst the TT2 says 0.3 sone and 54CFM, 1000-2000RPM. The TT2, Xtreme Plus and Xtreme 5870 are all rated for 250W and I find it hard to understand how the TT2 can do this with 54CFM, whilst the other two need 81CFM to do it (which must surely be noisier)?

In my humble opinion, AC just made some odd adds: each 92mm PWM fan accounts for 27 cfm and 0,16 sone (and the first figure is given at maximum speed, while the latter is with reference to the lower one).
So 2 fans give 54cfm and 0,3 sone (TT II), and 3 fans give 81cfm and 0,5 sone (XP II).
What do you think?

That makes sense.

I just don't get how the TT2 can cool 250W with 54CFM whereas the Xtreme Plus and Xtreme 5870 need 81CFM to do it.

Maybe you're missing the last part of the cooling "equation": how hot runs the card.
I think it just runs hotter with the TT II (54 cfm), than with the XP II (81 cfm).

However (with reference to what you heard about), if it's compatible you may go for the XP I, which seems to be similarly priced to the TT II, right now.

I guess you're right, it must just run hotter. It would be interesting to know how much hotter as then people could decide whether they're happy for their card to run that hot, or if they want to pay the extra for the XP II so that they can run it cooler.

I'm a bit nervous about my VRM, as it doesn't report temperatures, so I think I'll get the XP II as then I'm pretty confident it will be cool enough at 9v, which is essentially silent (about 19db I think).

It's not really worth me buying the XP I for £35 (including postage) as by the time I'd bought heatsinks and paste I wouldn't save much over buying the XP II for £45. I don't think the XP I has a 3-pin and 4-pin connector on the fan lead either, which the XP II does and will save me some modding as my card has a 3-pin header (although I'm not sure about this. One review showed this in pictures, but the specs I've seen only refer to a 4-pin connector).