Is this iMac enough for a web designer?

I'm sick of waiting for new iMac this year. So, when ever new one would be released, this summer, this fall, 2012... I don't care anymore. I need to buy an iMac 27" 2011 tomorrow and really need your suggestions.

Assuming that I will use this Mac 15 hours every day:

- Photoshop with 3-5 large files opening (20-30MB and hundreds of layers)
- Firefox 20 tabs (Flash, JS, large pictures...)
- Chrome 10 tabs
- Some small office softwares
- Dell U2312HM External Monitor

Do you think an iMac with this configuration is enough for the tasks:

- i5 2.7GHz
- 12 GB RAM (I will upgrade this later with an external 8GB RAM - $50)
- AMD Radeon HD 6770M with 512MB
- 1 TB HDD
- 120GB SSD (I will upgrade this later)

Any suggestion or comment would be much appreciated. Thank you.

 

I'm not sure but I'm using a Dell Core2Duo 2GHz, On-board GPU and 2GB RAM now and it's super slow when working with those PSDs (20-30MB web layouts up to thousand layers).

I need to open Photoshop, Firefox, IDE, Chrome, Word... at the same time. So a dual monitor setup would fit best.

I need your recommendations here which to choose now: an iMac with the configuration above or a MacBook Pro 2012?