Please help me update my graphics card, or get a new one that can handle Second Life. Here's my info, and?

Unfortuately, you motherboard only has the older AGP slot. Graphics cards for this slot are rarer and more expensive, but they can still be gotten.

However, seeing how they recommend at least a Radeon 4850, your only chance is probably the Radeon 3850 or the Radeon 4670. Their both pretty close to each other when it comes to performance, but while the 3850 is cheaper, the 4670 doesn't need as good of a power supply. Not only that, but AMD doesn't officially support AGP-based Radeon 4000 series graphics, so that can cause headaches when dealing with drivers (the 3000 series is fully supported however).

Other than that, just be sure that your power supply can handle your GPU, and that you're getting an AGP version, not a PCIe one!

Be aware that it's likely that these higher-end graphics cards may be limited by your processor if it's not a higher end model as well.