Just about the best for this budget.
PCPartPicker part list / Price breakdown by merchant / Benchmarks

CPU: Intel Core i5-4670 3.4GHz Quad-Core Processor ($214.99 @ TigerDirect)
Motherboard: ASRock H81M-DGS Micro ATX LGA1150 Motherboard ($46.99 @ Newegg)
Memory: Corsair XMS 8GB (1 x 8GB) DDR3-1600 Memory ($59.99 @ Newegg)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($59.99 @ Newegg)
Video Card: Gigabyte GeForce GTX 770 2GB Video Card ($329.99 @ Newegg)
Case: Thermaltake VL80001W2Z ATX Mid Tower Case ($21.99 @ Microcenter)
Power Supply: Corsair CX 600W 80+ Bronze Certified Semi-Modular ATX Power Supply ($49.99 @ NCIX US)
Operating System: Microsoft Windows 8.1 - OEM (64-bit) ($89.99 @ NCIX US)
Total: $873.92
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-01-31 18:26 EST-0500)
Do you want to overclock?

Well, I can tell you what I have in my computer and the prices on each if that would help you. My computer runs very very well, I have multiple screens and play multiple games at once, with no lag.
Intel i7 4th gen (300-330)
Nvidia GeForce 660 TI (200-300)
Asus Z87 Pro (300)
Water Cooling stuff ( In all about $400)
Level 10 GT Thermaltake Snow Edition Case (200)
1k watt Rosewill gold (170)
A crappy 1 TB hardrive ($50-75)
Twelve Gigabytes of RAM (Ran me about 100-200 can't remember exactly)
This is a pretty good build, I would like to improve my GPU and my Harddrive, but don't have the money currently.(Want a SSD) In total this would run you about one to two thousand, which seems like a lot, but trust me, when you build a computer you won't stop buying new parts.

Look at toms hardware gaming pc marathon

I am buying a new desktop too but i think i wont be able to build a desktop beacuse i dont know any pc part shops nearby. I can only order to build me one.

If you have $800 to spend you don't want to buy a prebuilt computer. They are generally overpriced or what they are. For $900 you could get a decent set up, and putting it together yourself is not at all difficult. The first computer I ever built, I built by myself with no instructions. This is just a suggestion for you. If you have any questions, or need any help feel free to message me.

900$ eh?

Well, I am guessing you live in USA. So I'd suggest this:
http://www.cyberpowerpc.com/system/Gamer_Xtreme_1000