welcome to the club! you seem like a nice guy!
about the mods, for performance the best would be a Rear sway bar upgrade. if you have a manual transmission car the you owe it to yourself to get a short shift kit with a counter weighted knob.
for looks i'd say some nice rims, fogs, mirrors, maybe a front lip, there are 3, stillen one, razi one and the AIT/Extreme-dimention one.
maybe a rear spoiler if you want (not my thing though)
http://forums.altimas.org/zerothread/301035
the eibach drop is always a nice upgrade but you can go lower with H&R springs or even more with Sprint springs.
there are some coilovers but those are a little more in the advanced mods category I think.
some nice mods could be dual tone leather interior
http://forums.altimas.org/zerothread?id=400197
I see that you wrote that your first mods will be rims and tires, I just want to warn you that the bigger you go (while keeping the stock overall tires and rim diameter) the harsher the ride will be.