But on the other hand, the upgraded heatsink is proving to be problematic. The old GPU heatsink was held down by a small bracket that not only clamps the heatsink, but also braces the mouse buttons on the keyboard. This bracket doesn't fit with the new heatsink in place - the heat block is too thick.
Also, the fan in the new heatsink makes a very annoying buzzing noise.
It's a shame really as it does do a very good job of cooling the GPU, but I think I'm better off with the original heatsink. The upgraded CPU is staying - a significant performance increase for all of £8.98 (including surprisingly quick postage) is not to be sneezed at.
Speaking of performance increase, as with all such upgrades it's the numbers that are important. So I've thrown PassMark PerformanceTest 8 at the laptop, and the results are in! Interestingly since the upgrade to Windows 7 performance has dropped slightly and the before score was only 233.9... but the after score is 259.2. The difference is more significant in other areas: CPU performance and 2D graphics both increased by around 20%, and memory got a 8% increase as well. Overall I think this was worth doing even if I am just about to take the laptop apart again to drop the old heatsink back in.
Oh, and Arctic Silver's ArctiClean is excellent for cleaning old thermal grease and pads - it merrily dissolves the stuff without any effort needed. Much better than isopropyl alcohol which doesn't seem to shift Arctic Silver 5 at all.