Can you run it on batches of 120 or so at a time? For me, with CS3 on Vista and 4G ram, PS slows way down if I have more than about 7 or 8 images loaded at once. If your batch can really handle 150 at a time, I would try the following:
1. Create 16 subdirectories and put about 100 into each.
2. Run the batch on the first directory.
3. Close PS and reopen it (because PS often doesn't give back used memory politely when it's not needed).
4. Run the batch on the next directory, etc.
Good luck.