Well, i think you know the memory's limits, you can't make a big, very detailed and fully skateable park.. :l
While I don't know how the THUG 2 engine works, I have been speculating on how to make the memory limit not as strict.
Simplified pieces to use less parts.
1. 4x4 Ground pieces. Why use 4 normal parts when you can use 1.
2. Long stretched out ramps, QPs, HPs
3. Large selections of the Block Parts. So 1x1, 1x2, and 1x3
At the same time keep the vertexs on the parts the same as there counter parts, but repeat the textures.
Also maybe a blank theme with no objects in the background would reduce the RAM used up?