Generating membership number

Hi all,
I have a question about generating membership numbers on the fly when someone registers to my website.

Rather than using the auto increment field as a membership number, I would rather keep it as just as the ID for the record and I would like to have a seperate membership number that looks something similar to this...

SR357324J

This will then stay with them for the lifetime of their membership and be on their printed loyalty card.

My questions are...
1) Is there a 'good practice' for membership number format and generation?

2) If this was used as a unique field, is there a degradation in performance when looking up records due to it being alphanumeric.

I may be well off base here, however these are my thoughts so far and your opinion/help is greatly appreciated.

Thanks for your contribution.I'm not sure about #1, but as for #2: You can still use the ID to retrieve records, even if you display the manually generated membership #.

If, however, you create your clustered index on the manually generated membership # field, you can make that field the clustered index (though this may hurt performance as relates to joins with other tables).|||I'd say let the db autogenerate an ID for you and you can append the 'SR' later..you could get into locking problems and duplicate ID's sooner or later..

hth