AGP and PCIe are different types of ports that graphics cards can plug in to. AGP is the old style and PCIe is newer (2004+).
I'm pretty sure the 3850 should be able to support Spore. I haven't checked the specs out yet. The main thing to look at in reviews is which version of DirectX it supports and how well. Sme cards I looked at were DirectX 10 compatible but did it really shit. I didn't mind though because I only wanted to be able to play DirectX 9 games.
EDIT: Specs for Spore are:
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PC: Windows XP/Vista, 2.0 GHz P4 processor or eq., 512 MB RAM (768 for Vista), 128 MB video card (Pixel Shader 2.0 support), 6 GB HDD. Internet connection. (For computers using built-in graphics chipsets: Intel Integrated Chipset 945GM, 2.6 GHz Pentium D CPU, or 1.8 GHz Core 2 Duo, 768 MB RAM)
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The only thing I'm not sure about is the Pixel Shader, but I'm fairly sure it would be fine. It's not a hugely graphically-intensive game from the look of it.