For a desktop machine, I would recommend a GTX 460, and in particular the 1GB or 2GB models, not only for the extra memory capacity, but the higher memory bandwidth over the 768MB model. Prices start around £120, and rise to £160 for the larger, beefier models.
For those with a little more money to spend, the GTX 560 Ti might be good choice - with an extra multi-processor over the 460 it gives a good 10%-20% better performance.
Choosing a CUDA suitable laptop is difficult, with a high rate of change in laptop design and development, and advice is typically out of date before it is given. Additionally, expectations for laptop chips should be much lower than equivalently priced desktop chips - typically 50% of the compute power. However, here are some recommendations