|"i quit updating my stuff. cause whenever i did it just ruined it"|
Miliions of people upgraded to SP3 without any issues whatsoever. And much of the current software requires SP3. You didn't do yourself any favors by not upgrading. If it "ruined it", it's most likely because you did something wrong, not because there's something wrong with SP3.
The 8400GS is a PCIe 2.0 card. It should work, but it may not. If you're not concerned with gaming & just want to get away from the horrible Intel graphics, something like this should be sufficient: