Im currently riding a Giant TCR Advanced(0) 2008 model with Dura-Ace 7800. my crank is a standard 53/39 Carbon FSA and I'm toying with the idea of replacing it with a compact. But not wanting to drop a huge amount on money on it.

I dont have a full appreciation of the cost and what is needed to replace my current crank with a compact and if I would get any value out of doing it.

I'm considering replacing it with a compact or mid compact to get a smaller small ring to handle the Six-Gap century in September and handle a few training rides in the coming months. I've been able to chug along with my 39-25, to handle 4-6,000 foot elevation rides but I'm concerned about those long climbs after logging 80-100 miles of climbing.

Would it be wise to ebay a compact ultegra/SLK for 200 orso? How do I made sure it fits my Bottom Bracket? Which bottom bracket do I have?

I can try to upload a photo or find a link later today if that helps.
Thanks,
v4e