Even if I do a transplant into a new case which I most likely will do, there wont be much headroom for large heat sync's, what one would you recommend?
I don't know what is available to you in your country, but I'm would hazard a guess that coolermaster's products are available. The majority of their products have 120mm fans attached and are no taller than 160mm, which will fit in any mid sized ATX case. They are not the best of their class, but should be available everywhere. They do get the job done and will always be better than a stock heatsink.
As a slight disclaimer, I found the zalman 9700 and have used them with every one of my builds since 2007. I keep up with many aspects of technology, cooling is not one of them. Simply replacing your stock heatsink with a similar model may solve all of your overheating problems. From what I can gather you do not ask very much of your CPU, it's just having some cooling problems due to dust build up.
What I can say is get something that is all copper. Even copper plated would do a better job. Just do not go with an aluminium heatsink.
I wish I could give you a solid recommendation, but it simply isn't an area that I'm knowledgeable in. I have experience with the 9700 and stock AMD and Intel heatsinks. There are many people on here with more knowledge than I have in this subject. I found a product that worked for me and wouldn't feel comfortable recommending anything that I don't have personal experience with.
The most advice I can solidly offer you is make another thread asking about cooling for AMD systems. If I was in your shoes, I would upgrade at this point. Between the overheating issues and your hardware starting to show it's age, it'd be better to upgrade than to try to make something work. However, I'm also understanding of the fact that not all of us have the funds to upgrade when we need to.
1)try cleaning your heatsink to the best of your ability
2)try underclocking and undervolting your CPU
if either of those don't work, then you really have no choice but to replace your current hardware. If after cleaning your heatsink you want to try underclocking and undervolting your CPU, I'd be more than happy to walk you through the process step by step.