Sparkplugs.com lists two oxygen sensors for our engine. I know there's "Upstream" (before precats) & "Downstream" (after precats). However, they list a "FRONT" (PN: 234-4045) & "REAR" (PN: 234-4610). I'm assuming that Front = Upstream and Rear = Downstream, but I'm not 100% sure and we all know what assuming does! Your input is appreciated...

EDIT>It should be noted that I'm looking at the "Denso" brand and I want to replace the upstream sensors.

Yes, front usually mean front of cat (or precat) and back usually mean back of cat (or precat). Bank 1 Upper and Bank 2 Upper have the same oxygen sensors. Likewise, Bank 1 Lower and Bank 2 Lower are the same.