How to increase the internal memory of HTC desire GSM?
300MB are the absolute maximum you can get without having an EXT4 partition on the sd card as a replacement.
Having a look at http://alpharev.nl/ you see that there's only a 330MB partition left, however none of my preferred ROMs fit that. It's made for some special ROM called Oxygen which I didn't like. 

With custom CM9/CM10 builds however I had to switch back to Bravo Stock again and I set up an external EXT4 1GB partition on my SD card. Those 1024MB are far better than having 30MB more (330MB instead of close to 300MB).