I'm not going to rundown your choices for Mac because...well you only have the one. If you end up deciding on a macbook, try looking at
refurbished one. Apple sells them directly, they're still well running computers and can be much less expensive. Don't get an Air, you're wasting money on screwball features.
I'm going to recommend Dell for a decent PC. I used one throughout college and it was pretty much indestructible. It ran well for nearly 6 years (I used it long after college) and I only stopped using it because I have a desktop PC now. You can also do better trades for price, whereas for Mac you're pretty much stuck with starting at $1000 new.
These are a good place to start.
I've been looking at a
VIAO for myself but haven't actually had the chance to use one. They look good, the specs seem good, it's a well rated laptop. For a portable semi-studio, they'd do a nice job.
These seem to rank consistently high too.
Just do a lot of research, decide what you need.
I would
strongly advise against getting a refurbished PC laptop unless you're getting it from a reputable source. Similarly, only get a refurbish macbook from a mac certified store or straight from the horse.
There's another reason I'm advising some of these computers based on price. If you're serious about using Photoshop (already expensive, get a student edition), then the other piece of hardware you're going to want to invest in is a tablet.
Wacom just released a stunning new
Intuos. You're not going to need anything larger than the medium, you could also go for the small. It's not a bad idea to get a used tablet if you want to cut cost. I used an Intuos2 up until....not even a month ago. They last a long time, there are a lot of series 3 up on ebay and Wacom still does support for older series.
Don't get anything with a serial port. Aside from the fact that I'm pretty sure any new laptop won't have one, Wacom also discontinued service for those this past Feb. There are no new drivers.