I was gonna replace the O2 in my DX/VX, I had sticker shock at the local Advance Auto ($340 if i remember right). A couple folks on here said go with a cheapo ebay O2, and replace it if need be in a few years. I paid $60 or $70 for one on ebay and its been working fine for me. I've for the most part, had good luck buying civic parts on ebay.
I've got a few mods, and have really tried to tailor my driving style to maximize mpgs... am around 54 mpg this tank (no EOC, and just starting to try P&G via engine-brake). I'd say for mods:
1) vacuum gauge (a must-have to keep the engine in lean-burn)
2) mpguino (well worth the $55, dcb on here sells 'em)
3) grill block (havent done it yet myself but everyone has good luck with them)
The biggest mod is your driving style. Keep it in lean-burn and you will improve mpg's fast. Try to avoid braking and use engine-brake downshifting to slow down til 2nd gear (then brake)... really helps mpg. I know this will probably get shouted down but (ahem) adjusting your driving location to (ahem) favor the low-pressure wake of a larger vehicle really boosts my 65mph mpg (even at a relatively safe distance)
