It's up to the league financial officials to work out the exact figure of Salary Cap and Luxury Tax Threshold for every season in July, normally announc it after first week of July.
For 07-08 season, salary cap was 55.63M, tax threshold was 67.865M.
For 06-07 season, they were 53.135M and 65.42M.
U may check out the previous seasons by google to figure out the increase rate. I'm not sure we can have a big one this year due to the cold economy. It is safe to say the LT line could be at most 71M.
There isn't really an increase rate for the salary cap or luxury tax. The salary cap is 51% of the year's BRI (basketball related income) for the league and the luxury tax lien is 61% of the BRI. It's probably safe to assume that BRI increased over the last year, but there's no way to really know how much until they release the numbers.
Joe