Ahmadjon :
You should assemble by yourself to make best available PC
And for what needs you want PC?
littleleo :
If you are experienced in choosing parts and assembling you own computer I would say build your own. That way you know what quality products are in your computer and there specs and warranties. You know what you can add plus you don't have to deal with bloatware and you can have setup just the way you want it. You can be sure the system is easy to upgrade or expand using standard parts.
When you buy a Dell or other brand you don't always get as good a quality in all the components. Plus the systems can be difficult to impossible to upgrade or expand if the motherboard or case or other hardware is proprietary. Plus they come with a ton of bloatware and may have some hidden surprises for you that can interfere with what you want to do at unexpected times.
Now if you don't know what you are doing then you nay have no choice but to buy a computer online. Of course you could go to a local computer store and give them a list of what you want and have them build it for you. Keep in mind they will need to mark up the parts and charge you for labor, no one wants to work for free.
I already have all the components sorted out. Would this be a good build?
Intel Core i5-4670K 3.4GHz Quad-Core $227.31 (CPU)
Asus Z87-PRO ATX LGA1150 $179.98 (Motherboard)
G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-1600 $75.76 (RAM)
Western Digital VelociRaptor 500GB 3.5" 10000RPM $128.02 (Storage)
MSI GeForce GTX 660 2GB $194.99 (Graphics Card)
Thermaltake Chaser A31 Snow White ATX Mid Tower $65.76 (Case)
Corsair 750W ATX12V / EPS12V $114.98 (PSU)
Asus DRW-24B1ST/BLK/B/AS DVD/CD Writer $24.84 (Optical Drive)
Microsoft Windows 8.1 Pro (OEM) (64-bit) $151.53 (OP)