You won't have any issues mixing AMD and Nvidia, but if you pick another Nvidia card, you can manage all the features of both cards through one driver control panel. It's a minor point but it's useful for things like Adobe colour settings and ICC profile management.
If you're not planning to game, something like a GT520 is probably your best bet:
- It runs the exact same Fermi architecture as your GTX470, meaning any driver/application quirks you've had to workaround with the GTX470 will also be fixed on a GT520
- It's really cheap - as low as $25
- Most of them are a short, low profile card design to avoid blocking the air-intake of your GTX470's blower
- It only consumes a maximum of 29W so you don't need to worry about your power supply, PCI-E power cables or extra airflow
- It's another 48 CUDA cores going spare, if you use any CUDA or OpenCL applications in your work.