Computer vs Laptop/Notebook:
Unless you know for sure that you will live in the same place in 2 years, you should always go for the laptop/notebook. You are more flexible and can simply buy a big monitor and plug it into the laptop.
Worst mistake: Building together your own computer
Even if your friend who is an “expert” advises you to buy all parts and put them together later, do NOT listen to him. When you will have troubles at 3am on a soft table or in the middle of a tournament, he will NOT be able to help you.
You will not have any guarantee to return the whole thing and get a new one. Your friend will also stop being your friend after you have called him 5 times the last week.
How to buy a computer and monitor the right way:
If you are a dedicated gamer, then you will not need my advice. If you are a “normal” guy like myself, just buy a computer/notebook that works. Computers become faster, but poker site software does usually not eat a lot more resources. Buy something that works. If you have the extra money, invest in speed.
Cheap usually works out well as long as it is new. Do not buy used electronics.
Of course, if you can afford to pay more, do that. You are likely using your computer every day, so buy something good if your financial situation allows it.