I'm looking at getting an Asrock 880GM-LE FX or 880GMH/U3S3 (preferably the former as I can only find a few s/h or refurbished 880GMH/U3S3 and I'd rather buy new for the warranty). I need to fit a HDT-D1284 CPU heatsink on there but I'm rather concerned that the height of the heatsinks on these two boards won't allow it, as looking at this image it doesn't seem like there'd be enough space as the Asrock heatsinks look higher than normal.

So I'm wondering if there's some suitable some third-party solution that's reasonably cheap I could replace the NB heatsinks with? Whatever I use, I'd need to use the mounting points to secure it as I'm going to be shipping the PC so can't rely on adhesive holding it on!

I think you would be better off finding a similar spec motherboard - hopefully at around the same price - that does not have this problem. One possibility is the MSI 880GMA-E35 from sources such as this in the UK http://www.scan.co.uk/products/msi-880gma-e35-(fx)-amd-880g-s-am3plus-ddr3-sata-iii-6gb-s-sata-raid-pcie-20-d-sub-dvi-d-hdmi-matx.

Cant the HDT-D1284 be mounted in any direction?,
if so just mount it in such a way as to miss the heatsink, maybe with the curved part of the heat pipes facing towards the VRM's

There are (were?) three different heatpipe cooler series for the chipset:
Thermalright HR, only one I can currently find one with two heatpipes HR-55 (you could probably cool a 35W CPU with that).
It installs with steel hooks or screws, so it should be safe to ship, but it is expensive ~£18.
I used the HR-05 on an older (hotter) chipset. It comes with push-pins which are definitely not shipping safe.
The Deepcool/EKL version uses push pins, too.
THe Xigmatek Porter uses Screws and is fairly inexpensive @ ~£9. I think in part because it's EOL?
I don't now the (r)etailer I linked to, the shop came up on a price search (skinflint).
BTW, Unless you go full passive, a heatpipe cooler for the chipset might be overkill.

No, it seriously drops in efficiency if it's mounted in any other orientation other than with the ends of the pipes at the top (i.e. the curves of the pipes at the bottom). I've read this from multiple people who've used/tested it.

Sorry, you've misunderstood me. I'm not looking for a heatpipe NB cooler, just a lower heatsink that won't clash with the CPU heatsink.

Enzotech make copper northbridge heatsinks which can be a lot lower but they are not cheap - for example http://www.specialtech.co.uk/spshop/customer/Enzotech-Northbridge-Heatsink--CNB-S1L-pid-12848.html.

Zalman makes/made a couple of Northbridge heatsinks. ZM-NB47 and ZM-NBF47. Perhaps there's a UK vendor that still carries them...

So you think it will be fine to replace it with a lower one and that there's no good reason why they ones already on there are so high?

I might even have a heatsink I can take off a faulty board.

Copper heatsinks should be more efficient than aluminium ones, so can be smaller in height. There are two versions of the Enzotech CNB S1 cooler, the L model being 11.6mm high and the other 27.6mm. The higher one might be a safer bet and due to its construction it would be easier to deal with any snagging. However at £13.59 it is even more expensive http://www.specialtech.co.uk/spshop/customer/Enzotech-Northbridge-Heatsink--CNB-S1-pid-12847.html.

I have replaced a northbridge heatsink using the Zalman ZM-NBF47. The Zalman is aluminium and is significantly cheaper than the copper Enzotech models. The issue with the NBF-47 may be its height, 47mm as its model designation suggests.

You could just use a lower height aluminum sink from elsewhere, as you suggest. It might result in higher northbridge temps but if the system is never going to be particularly highly stressed it is unlikely to matter.

Thanks lodestar. It's only going to be used as an HTPC, so the IGP/NB will only be decoding video, no games, so hopefully it won't get too warm.

I'll have a look at those heatsinks and see if I've got any copper ones already. The stock heatsink on the 880GMH/U3S3 is considerably wider but I guess they all use standard spacing for the mounting pins so it shouldn't matter too much.