IMHO, you wouldn't be tied to a desk 24/7 with a "small" but powerful 13" MBP since it is portable & probably "light" enough to carry everywhere you want.
Based on your budget of $1,500, I humbly think that the best option right now (given the available choices) is a 13" base (2.4GHz) MBP. The part number, if you want to shop around online, for this MBP is
MC374LL/A. The best price for that particular model brand new is
$999.99 (price is before sales taxes) at MicroCenter. It will be great if you have a MicroCenter near you because they only sell them in stores.
In regards to the other part upgrades, I'll suggest that you wait & see how it goes when you've used your new MBP for a period of time. If you find that you need more memory or need more storage or a faster HDD/SSD, you can always buy those aftermarket since they're cheaper. At this point, you're probably gonna have slightly more than $400 left over after your base 13" MBP purchase, depending on how much state taxes you have to pay at your local MicroCenter store.
For about $400, there are quite a number of choices to choose from in regards to a computer monitor. Since you're doing lots of Photoshop stuff, I can be sure that most people around this forum will agree with me that you'll probably want to go for an IPS monitor, instead of those cheap TN monitors. For the best bang of the buck as of today, I'll like to suggest the
Dell U2311H UltraSharp 23-inch Widescreen Flat Panel Display, which can be had for about
$279 (price is before sales taxes). It is an IPS panel with a high enough resolution of 1920 x 1080, which will be fantastic for photoshop activities.
With the purchase of an external display, you'll definitely want to go ahead & purchase an
Apple Wireless Keyboard. As for the mouse, you may want to consider the
Apple Magic Trackpad, the
Apple Magic Mouse, or even a wireless mouse made by another company such as the
Razer Orochi Bluetooth mouse. That is going to make it easier & more convenient for you to work on your MBP when it is hooked up to your external monitor.
You might also want to consider buying a few not too expensive but useful accessories such as a notebook sleeve for your new "baby", the display adapter/cable to connect your MBP to the monitor (buy this from a third party company such as MonoPrice), & so on.
The above are just my suggestions.
I'm sure fellow members will be able to give you better advise & suggestions.
Good luck.