the engine I have now is a piece of crap. Im looking to buy a new engine that'll give my '96 altima the most horsepower. Im new to soup'n up the cars and stuff so if yall can help a dude out then feel free  

So nobody knows anything huh? Thats cool....thasnks anywya guys  

lol give it a couple days dude im not a nissan guy so i cant really help but i know a little so ill say maybe look into hmm...well i donno i wanna say SR20DETT but i feel that engine is smaller than a maxima's engine(i could be wrong)but it gives you something to look into lol  [some better advise sell it and buy a new car lol honda cough]  

I've been working like a horse and only come on every few days. Forgive me for living life! LOL!
I like Nissans a lot. The Altima in question has a good engine, just not very powerful... You could beef up a SR20DET and install a decent turbo... Costs some money, though. The KA20 engine is also a good upgradeable engine, it's a steel block that can handle a little pressure. Just don't down grade to an older engine. The laws are strict about that due to emissions laws that are in effect. You will have to swap out the PCM or get it reflashed, if possible, and wire harnesses will need to be modified to accept the changesd made. All this is to make the engine legal to the chasis through VIN verification in the PCM.
Your drive line will also need to be inspected and upgradedas well, if needed. More power means more stress to the drive line. That also means that the transmission will need some attention to detail.
Getting more horses means taking the whole assembly into consideration. It's like building a house. If the foundation is not built right, then the whole house willl be wrong. Understand? All that means that you will spend a lot of cash. If it's a daily driver that you want with a little more "umph", then just rebuild the engine it has and have some performance touches done to it. Example; Bigger valves with performance specs, performance pistons and rods( maybe stroke it), port matching on the intake and exhaust sides of the engine to the manifolds, headers that compliment the build with exhaust, performance high flow( not high pressure) oil pump, performance throttle body, cold air intake, performance radiator, Turbo, if it's in your budget... the list is long but you see where I'm going with this, It's still a lot of money. Swaps are great but you have to out weigh the cost of doing one over the other. The engine that's in there already has some potential, you just have to upgrade. Again, it will cost you some money and it will leave youwalking for a while till you finish the job. I hope this bit of insight will help you decide on what to do.  

I wonder if the 3.0 fits in there?  

With some modifications to the front clip. It's kinda' tight.