A tracker would open a lot of doors, not just for things like higher resolution lunar imaging, but astrophotography in general.
You could then push that lens you have to the next level (Minolta 400, F4.5). You can get a lot more resolution out of that aperture, that's 88mm aperture, that's practically a 90mm refractor telescope, literally. You can take that lens to F18 with two 2.0x TC's and it will not lose potential resolution. Your camera's pixel pitch is 4.5um. So, around F22 is where it will top out for sampling. Longer than F22 will over-sample (gain nothing). Undersampling (shorter than F22) is ideal here. F18 puts you right in the sweet spot to get as much resolution out your lens as you can for your pixel size on your camera. Again, that Minolta is basically a 90mm refractor.
It's just not easy with a handful of still images. If you switch to using your dSLR for video you will get hundreds or a thousand frames much faster, to beat the seeing (imaging no slower than 10ms exposure time to freeze the atmospheric seeing). Imaging at 10ms (1/100s) as your slowest exposure time, push ISO to whatever it takes to fill your histogram and stacking to restore signal to noise and reduce random noise.
WOW! Thanks SO much Martin! THIS is the info I have wanted to hear for quite some time - how to get the most out of what I have. I have been wanting to try two 2X TC's... but was unsure whether it would gain me anything.... and I have read about using video - nice to hear I can try it! So... a tracker it is. Won't be any time soon, but it is "next". Have to start thinking about a budget now... then look at the options. If I get itchy.... well.... I don't use my 300/2.8 all that much - I could always sell that. Wait..... you will probably tell me I should be using THAT lens right? Its deadly sharp.... but absolutely horrible CA in spite of being an APO. hehe