Empire

First things first, decide if you truely LOVE the car. Build a sentra can be done. And has been done. We used to have a member on this site - RIP, we miss ya bud - that built a pretty nice, rather well performing b15.
The catch is, it isn't going to be as easy as building a honda, or a 240, or any of those really common cars because typically the kids that own sentras don't have the know how, understanding, nor the budget, to actually build a nice car. You'll find most of the sentra forums full of kids wondering what wheels look the best and trying to find cheap ways to keep their cars on the road.
with the 1.8, you're options for power are pretty limited, either an engine swap, or piecing together a turbo kit.
You can do things like intake, headers and exhaust, but don't expect a ton of results out of it. Maybe 5 with the intake, 10 with good headers, and maybe another 10 with exhaust. The engine just wasn't made for performance. However, if you throw enough money, and knowledge at it, you can make good power with a proper turbo kit. It just isn't going to be as easy as say, boosting a 240sx would be. In the 240 world, you can pretty much google the recipe on how to make power. But with sentras, you're going to have to educate yourself on how engines work, and how they actually make power and all of that "lost art/black magic" of engine building that so few have today. If you love the car, it could be worth it. You'll learn more and could end up with a decent car when it is all said and done. But, it will take a ton of time, and a ton of money.
the other option is to swap in a difference engine. Some people will go for the 2.5 liter (QR25DE) found in the SE-r's and spec Vs. They are decently capable, and it does give you more displacement to play with, but is again, going to take a lot of work to build one up nicely. Around here, the preferred option would be the 2.0 liter, (SR20DE) as the sr20 is a much more commonly known engine. You could more than likely swap the engine, bolt on some turbo parts and make more power with a more conservative build.
Beyond that, I would focus on suspension and drivetrain. The spec v came with a helical LSD, and a 6 speed transmission, and pretty decent suspension set up. All of which would be a huge improvement over your car.
SR20DE(T), conservative build, 6 speed tranny, helical LSD, upgraded suspension, cosmetic improvements, decent wheels, upgraded interior, decent sound system, and you'd have a pretty decent car.
again, if you have the time and money for it.

Depends,
- What goals do you have? - you just wanting a car to work on to learn about cars and performance? Or you wanting to build a super fast drag car? A sweet ride to go cruising around in? A car to set economy records? a peppy daily driver? The biggest bang for the smallest buck? If you aren't picking a car because you simply love that car, then you need to put a lot of thought into what you are actually setting out to accomplish.
- What kind of budget are you working with? Do you have a stack of cash right now? Are you broke as a joke right now? Do you have the kind of income that you can put towards a car over the next 6 months? 12 months? 18 months?
- What kind of car requirements do you have? import, domestic, fwd, rwd, common, rare, different, new, old, modern, classic, etc.

Nothing sucks worse than trying to build a car and not knowing what you actually want. You end up wasting a ton of time, and a time of money and since you don't have any passion for what you are doing, instead of going out and educating yourself, you'll end up listening to anyone who has an opinion, and not even know if they know what they are talking about, and you can end up with a broken pile of parts. I'd rather light my cigars using 100 dollar bills instead of building a car with no plan, or even goals. At least then, I'd be enjoying throwing money away.