So many questions...
What are the numbers that follow some of the hardware like DDR 2100, DDR 2700, P4 2.8Ghz, P4 2.8GHz 800Mhz?
You will never see anything like "DDR 2100"
But more likely, you will find:
"PC3200 DDR400"
This means that the type of RAM being used, DDR 400 runs at 400MHz.
For PC2100, it will run at 266MHz.
For PC2700, it will run at 333MHz.
As for the CPUs.
P4 2.8Ghz, P4 2.8GHz 800Mhz?
The 800MHz signifies the Front-Side-Bus (FSB) frequency.
In this case, it would be the Pentium 4, with 800MHz FSB (HyperThreading P4 "C"; for Canterwood CPUs will be running at 800MHz).
All other Pentium 4's will run at 533MHz (the older ones).
In your particular situation, I would not want to create more questions than is necessary - so as to not confuse you, creating more questions, and so on.
Do i need to worry about it or can i go for the cheaper one?
PC2100, DDR266 should be fine if you're working on a budget.
Bare in mind that if you purchase motherboard that supports PC3200 (for example, DDR400) - when using DDR266 RAM, you will be reduced to 266MHz, instead of having the "right" type of RAM - running at 400MHz.
The good thing about buying DDR400 or higher RAM is that it is backward compatible and can be "underclocked" to run at slower speeds, on older boards.
So if I used my DDR400 RAM on a machine, such as Winston's running an AMD CPU, I would be able to use my faster-rated RAM (at 400MHz) in a slower system, be it 266MHz or 333MHz.
You should not worry too much about this, unless you are willing to read a lot on your own, do your own research because I will not be here to hold your hand and spoonfeed you, guiding you step-by-step.
Search the Internet and read plenty of reviews, learn the basics before you attempt to find out the specifics or read technical documents.
That is all I can really say, you shouldn't worry too much
right now - if money is an issue, then go for what is best suited for you, and your budget.
Ask plenty of questions, but don't rely on others because ultimately - you will have to make the purchasing decisions on your own, for your own personal needs and uses.