Gringo Vermelho is correct, you can adjust to match your native resolution; however, you will not be able to obtain hardware acceleration. This is a prerequisite of many applications.
The graphics card you purchased is not supported on 10.5; however, as JavaKrypt said, it is supported under 10.7 Lion. If I am not mistaken, it may also be supported under 10.6 Snow Leopard.
Please provide us with some specifications about your system (CPU, Motherboard, Graphics & Model Number if applicable). Adding more specific information allows us to respond quickly with more precise answers. Chipset, LAN, Audio & If your Graphics are integrated or not, are excellent additions.
You can also place this information in your "Signature". It will then appear on every post you make. This can be done via My Controls:
http://forum.insanel...p;hl=signatures
Please follow the rules described in this thread (Most importantly keeping your signature to no more than 5 lines of normal text:
http://forum.insanel...p;hl=signatures
With this information, we can point you in the right direction.
In the future, you may want to determine hardware compatibility prior to purchasing. Make no assumptions.