What is the difference between xbox 36 4gb and the one 250gb? Which one is the best one? Is 4gb enough? How do you know if the xbox you are buying is 4gb or 250gb?
Well, the 'gb' (which stands for Gigabytes) is just the amount of space the hard drive has. The bigger the better in my opinion (if you can afford it). 4GB will not be enough, especially if you're planning on downloading games to the Xbox. Most games are 4GB plus so just installing one game will wipe out your hard drive space. 250GB is obviously more expensive but I recommend it as you won't need to go out and buy a bigger hard drive in the future. On the box it will have either "4gb" or "250gb" in big writing, that's how you know what Xbox you're buying. Also it will be in the description of the Xbox, so read carefully what you're purchasing. As I said before, the bigger the hard drive the more money you will have to pay; but it's totally worth it! You need hard drive space to save your games progress, download films, games, music and exclusive in-game add on's/content.
I thought you buy CDs from a marketplace such best buy or so but I have seen forums in which people talk about downloading the games. Is that game the same like the one that comes from the CD? Are downloadable games cheaper than the CD ones?
You can buy "CD's" from a shop and use them just like you use a PC game CD. Almost all games you need to buy the disk for from a shop such as Game or Walmart (ASDA). However, certain games are available for download from the Xbox 360 marketplace. This is because they're either older games which Xbox have decided you can download them or some newer games which don't exist on a CD. They're not really cheaper as Xbox sells them for the value which they would be in the shop. It really depends on the game to be honest.
What are xbox points and gamertag? What are they for?
When you say Xbox points I think you mean "Gamerscore"? This is just like achievements you build up when you play games. If you unlock an achievement by performing a certain action in a game you will receive Gamerscore which adds to your profile. It doesn't really mean anything and can't be used for anything, it simply shows your progression on that game. DON'T confuse this with Microsoft points. Microsoft points are purchased with IRL money and are used to buy downloads, games, music and other in-game addon's and features.
A Gamertag is essentially you online username. Just like your Sythe username, it serves the same purpose; it allows others to see who you are online and your friends can identify you by your Gamertag. Gamertags are unique and there will never be more than 1 of the same Gamertag on Xbox LIVE; So chose carefully! If you want to change your Gamertag it costs 800MSP (Microsoft points) which costs around £5 or $8.
can you play all games online?
MOST, yes. You can tell whether a game has online features because it will have a grey banner at the top of the game case which says "Xbox live". This means it has online features. Also if you read the back of the case you will be able to see whether or not it has online features. If you need to know whether a game is playable online and can't find out yourself just PM me. I'll be able to tell you
Is Kinect really worth it?
From personal experience, no. I got mine for christmas a year ago and sold it shortly after as I didn't enjoy it and there wasn't enough games out for it. It all depends on personal preference as many people enjoy using the Kinect. There are many more games available now than when I had mine so it may be awesome. You'd have to try it or see some videos of the Kinect in action and decide whether you like the look of it or not

Many new games such as "FIFA 13" now have Kinect capibilities so that may be interesting. My best advice would be to take a look at the games which are currently out for the Kinect and games which have the Kinect capability. If you like the look of the games then you're most likely going to enjoy using the Kinect
I hope those are the answers you were looking for and if not just send me a PM! I will always reply back and will help you to the best of my ability.
Xbox's are great and are totally worth the money. They're really enjoyable and I love mine; It's great to interact with friends and there's some really good games out.
EDIT, A COUPLE OF POINTERS/TIPS;
- You need to purchase "Xbox live" to be able to play games online. It's around £35 for a 12 month subscription which is around $50 maybe? You can get shorter subscription times but they end up costing more per month.
- Shop around! You can get MANY different bundles with different features. So shop around, seriously. eBay offer great deals with many trusted sellers. eBay can be up to 20% cheaper than most retailers in some cases.
- You can install the game CD to your hard drive; however you still need to insert the disk even though the game is installed on the hard drive. You install a game onto your hard drive because it generally runs the game smoother and your Xbox doesn't need as much processing power which means it doesn't get as hot and sounds quieter. Another benefit of having a bigger hard drive space.