Mac Pro vs my old Windows computer

[Sheiky]

How much, if any, performance increase or decrease do you think I would get by purchasing the macbook pro 13 inch monitor compared to my old computer.

Little increase I think. Definitely don't get your hopes up too high.

The traditional Hard Drive is the slowest component in todays computers. In your case you are used to a 7200RPM hard drive, this is most likely faster than the Mac's 5400RPM drive. The Mac wil have a faster CPU, but the speed of the hard drive is the "speed you feel".
Having a fast processor and loads of memory sound really nice, but the point is that you hardly use it. Having 8 or 16GB of RAM sounds amazing(and it can be) but truth be told is that you really have to push your machine to use it. For normal use and Lightroom plus the occasional edits in CS5 4GB will be fine. If you have more you are likely to use it only in small peaks. Processes which consume a lot of RAM are for example stitching multiple shots or using heavy filters in CS5. Or if you do real heavy multitasking which I doubt you will on a 13" screen. With more than 4GB these processes will just take a little less time on 8 or more GB's. Is it worth the extra money, that's up to you. It is cheap at the moment

As I said the traditional hard drive is the slowest component in todays pc's. If you want a faster PC go for a Solid State Drive. With a Solid State Drive you can take more advantage of a faster CPU and more RAM, because the drive is fast enough to enter lots of data simultaneously. SSD's are currently quite nicely priced compared to the traditional hard drives, just a thought

As far as Dell vs Mac is concerned. If you know how to work with W7 and disable those useless Dell apps that boot every time you start your pc, I guarantee you there's practically no difference in speed between W7 and Mac. It's a myth that you can't get a virus on a Mac or that Windows is slower than Mac OS. Just like bigblue1ca says: it's like Canon vs Nikon. Both have their pro's and cons, but all in all they pretty much give the same result.

My advice: go with what you like. Mac can be nice(I personally don't like the workflow) and it might be more easy to synchronize with your wife's Mac, iPads, iPhones and all other i-stuff. A new Dell or any other brand W7 laptop will do the same tasks just as well. Somewhat cheaper which is the most important factor in pretty much any Mac vs Windows debate and usually the warranty is better as well.

Ps: a new pc will always feel faster and better so enjoy it