You shouldn't lose more than 2lbs / week if you want to stay healthy. That means about 16lbs by 1 Sept 2012.
I know i's hard to wait for the results but losing weight slowly is more beneficial for you in the long run. It's more likely you can keep your weight down permanently, less likely you get malnourished, and less likely you get saggy, loose skin at the end.
Good luck, and let me know if you need any help.
What you eat can have a direct effect on your skin if you're struggling with psoriasis. See what to shop for.