Core 2 Duo 6600 (2.4 GHz, 4MB cache)
2GB RAM
Debian Linux, kernel 2.6.34.1
BitCoin 0.3.0 Beta (32-bit x86), with custom SSE hash code

Very responsive. No significant change in interactive response.

1 thread: ~4200 khash/s
2 threads: ~7700 khash/s

EDIT: For reference, I was getting about 875 khash/s (two threads) with the stock client and platform-specific compile options (-O3 -march=nocona -msse -msse2 -mfpmath=sse,387). Reorganizing sha.cpp, within the bounds of plain C code, pushed that up to ~1000 khash/s.