nvidia gforce 7300 gt 256 mb

You can install 10.7 Lion with Apple's blessing, (your tag line says you are running 10.5.8) and later versions with some unsupported (slightly complex but reportedly stable) hacks.

Regardless of what version you want to have eventually, all paths go through 10.6.8, which includes [Mac] App Store.app (once it is fully updated with Software Update). The DVD required to install 10.6 is available at the Apple Online Store (not the App Store) for about US$20.

The unsupported hacks are not inherently troublesome by themselves. The problem is that you need to be able to tell when any problems you encounter are caused by "regular" issues or by the hacks. If you are strictly a creative type, you may want to avoid them, as they could force you to learn far, far more than you ever wanted to know about the insides of your Mac.

The way they work is by using the 32-bit firmware routines already in your Mac firmware, and adding additional 64-bit routines required to make the complete 64-bit set. These additional routines do not fit in the firmware store, so they are executed from main RAM memory. (as such these cannot be used to run Mac OS X on brand-X hardware.)

http://oemden.com/sixty-four-on-thirty-two-sfott/

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You have to order Lion here:

http://store.apple.com/us/product/D6106Z/A/os-x-lion

To go higher

(Unofficially, can run OS X 10.8 "Mountain Lion, 10.9 "Mavericks" and 10.10 "Yosemite" with an upgrade to a supported graphics card and EFI64 emulation)^[30][31]