need advices about new Comp?

GT 550 Ti? I think you mean GTX 550 Ti. I own a GTX 460 (which is also what I think you mean by GT 460) and the 550 Ti would perform better. But for a great performance, try out the 560, or if you can afford it, the 560 Ti. Radeon does make the better graphics cards (the competing cards, say the 460 and 5830, Radeon usually has the better one), but they are both fast, so don't worry about that.

The i5 is better than the i3, however it matters on which model of the i3/i5 you choose. I would suggest the i5-2500k (make sure the motherboard you choose has an LGA 1155 socket type). 4GB of RAM is enough, but if you want to make it last a bit longer, try 6GB (DDR3, 1333 MHz). 500GB of HDD is good, as long as you're not going to store a whole bunch of pictures, videos, music, etc. in there. I'm assuming that you're building this for gaming. That's all you need to know, and happy gaming on your shiny new computer.

(Jealousy...)

Start off by getting good info on motherboards, GIGABYTE botherboards are fairly powerfull and cheap, some of them can have up to 32gb of RAM, with dual PCI for double graphic cards, dont forget the smaller needs: powersupply, cpu cooler fan, optical drive, this things are easy to get and most of the time cheap but they still may add $100 to ur original price... Also the HDD doesnt only need to have tons of memory but also a fast rate of reading data. Get a 200 gb of memory and 6GBPS (gigabytes per second) of speed and u have a good hardrive. Last but not least, go with NVidia, radeon doesnt have as much support and high tech stuff as NVidia, good luck!

You don't give quite enough information. The first and most basic question is what do you use it for? Without knowing that, you could waste money on one thing that would be better on something else.

For example, for average use the on chip graphics on both intel and amd processors is adequate. If you do gaming they are not. Don't just get a GT550 for example because the card the chip is on is just as important. For example, XFX, MSI, Asus. The present range of processors, Intel perform far better than AMD but again, what are your needs?

I have recently built an i7 2600K system for a customer, the parts alone costing £1300. What a waste, it is used for internet and basic office work. At the same time I built an AMD Phenom II x4 system for another customer, same sort of use, £316.00 for parts. A sensible buy for his needs

Go for what you need, not big numbers.