I was in a similar situation about 5 years ago (out of college, lived at home, no expenses). Went for the new car. Then I moved out (no roommates) six months later. Not too long afterward, I had got used to a lifestyle that consumed all my income (surprisingly easy) with nothing left for savings/retirement. If I had started maxing out, say a Roth IRA, I could have had nearly $20k as a down payment on my house a couple months ago; and that would have cost me about $15k in after-tax dollars... You can do interesting things with 401(k) and such too...
Now that I have a 103% mortgage, I have to pay PMI and a higher rate, so my house costs a lot more than it needs to (about $240/month more than with $20k down). I still have exactly $125 in all forms of long-term savings. And my wife is about to go on maternity leave, but my income will not cover our expenses. And we need a new car. And she had a knee injury while pregnant that will require surgery (more $$) and more down-time from work (more $$).
The future seems far off, but a lot of things can change in just 2-3 years, and the decisions you make now can have a huge impact on your quality of life when things change (and they WILL change).
Run the Sentra into the ground, max your retirement, save up. You'll have a whole lot more options open to you (better cars, better homes, etc).