New member here.
Sorry I haven't done a formal intro yet, but I really want to chime in here.
I come from a video editing background, and I'm a long time Mac user.
I wanted to comment on one of the most important yet most overlooked aspect of using a Mac for any type of imaging work.
Which is system wide color profiling.
Unlike other systems, MacOS X has built in system wide ICC profiling.
This means that images maintain a constant appearance through all Apple apps and 3rd party applications.
Windows systems are only color managed when using an application that uses color management such as Photoshop.
So on windows, the images you see in an explorer window are NOT color managed, meaning they'll have a much different appearance in explorer than in photoshop.
In contrast, images in a finder window on a Mac will look exactly the same as they do in Photoshop, Aperture, and/or iPhoto.
Do you not have a computer that you work with images on right now at all?
What are you posting here with?
Because honestly, as has already been said, a budget of $500 for a system to work with images on is really not advisable.
That entire budget would still be on the low end for the display alone.
I think even a 20 inch Apple Cinema Display is like $800.
I'd personally hold out until I had more $$$ to work with, sell some stuff you have or take a 2nd job and put it all in the budget for a new machine.
Have you considered a Mac-Mini?
They're not as expensive as other Macs(I think they're $599 for just the unit with no monitor mouse or keyboard) but still offer the same benefits.
They aren't as powerful as a MacPro or MacBookPro, but they're fast enough to get you started in building a client base and workflow that will allow you to generate income you can sink back into the project to get a more powerful system at a later time.
Then you have a seperate machine to run something like QuickBooks on to get yourself even more established.
Add a 'Time Capsule' later too, and you have an all in one backup solution for the Mac-Mini running quickbooks, and your new workstation with all your assets on it.
You really don't want all your eggs in one basket.
I learned that the hard way with the PowerMac G5, which all but completely died on me before it was a full 4 years old.
All my assets were on it, and it set me way back in productivity and money.
I wanted to blame apple, and I still believe they are somewhat at fault, but in the end I was the one who put all my eggs in that basket.
I'm still recovering from that.
But in trying to see the positive, that experience forced me to create a good backup policy.
I created automator actions that copy images to both Aperture library and to network attached storage devices simultaneously everytime I dump a card.