MAC makes a good product, but they are very proud of it... I do a lot of photo editing, but I use a Desktop system for the bulk of my editing. I do have a laptop that I do some editing on and it works for me just fine. I run photoshop cs5 and Lightroom 3.
I see three options for you..
1. If you are totally in love with MAC and do not mind the price tag, go for it.
2.. Laptops are the ONLY pre-built computers I will purchase... and I will only look at 4 brands.. IBM, Lenovo, Toshiba, and ASUS. Everything else is crap.. Yes, even Dell.. .. I know this because of the experience of being a pc tech for over 12 years. Rule of thumb when purchasing a laptop.. Never buy bottom line or top of the line.. stay in the middle. Plan on spending anywhere from 600-1000 on a good laptop. After purchasing one the first thing I do is wipe it clean and re-install the OS to get rid of all the crap-ware that is installed. PITA, but fact is... the system will run quicker. Make sure you get a 64 bit OS to take advantage of the memory and your dual core proc.
3. If you are really wanting a MAC but can't afford it.. buy a Laptop and wipe it and then install the MAC OS on it.. Heck, you can probably do this to your Dell.
People using MACs and people that work there will try and tell you that it will not run as well and that it is not designed to do that, but fact is... a computer is a computer.. If you build out a MAC on their site I can build a PC for 1/3rd the price using the SAME hardware and then install the MAC OS on it and you would not know the difference. The price tag is in the MAC OS. Now, you cannot "Legally" obtain the software and install it, but I have my ways.. If interested, PM me.