10.2 is definitely the best option if running X at all... 10.1 will be painfully slow.

The 5500 came with OS8 (the original variant) when new, however retail copies of 8.1, 8.5, 8.6, 9.0 and 9.1 will have full driver support for it... Of those, I would recommend either 8.6 or 9.1 as the most stable and functional systems, with 8.6 being faster but 9 having a bit better support for certain later hardware and software.