Well, I can answer most of your questions and I can show you what I just ordered (shipped today) for around the same budget with keyboard, mouse, & Win7 but I already had a decent monitor.
First up, Overclocking is the process of running a computer component (generally CPU, but can also include the RAM and video card) at a higher clock rate (more clock cycles per second) than it was designed for or was specified by the manufacturer, usually practiced by enthusiasts seeking an increase in the performance of their computers.
Second SLI: Scalable Link Interface (SLI) is a brand name for a multi-GPU solution developed by NVIDIA for linking two or more video cards together to produce a single output. SLI is an application of parallel processing for computer graphics, meant to increase the processing power available for graphics.
And Crossfire: AMD CrossFireX (also known as CrossFire) is a brand name for AMD Graphics Product Group' multi-GPU solution. The technology allows up to four GPUs to be used in a single computer to improve graphics performance.
Sum it up: SLI is Nvidia's solution to using multiple graphics cards and Crossfire is AMD's solution.
For the Budget, I would go with an SSD for OS and Main Programs
A solid-state drive (SSD) is a data storage device that uses solid-state memory to store persistent data with the intention of providing access in the same manner of a traditional block i/o hard disk drive. SSDs are distinguished from traditional hard disk drives (HDDs), which are electromechanical devices containing spinning disks and movable read/write heads. SSDs, in contrast, use microchips which retain data in non-volatile memory chips[1] and contain no moving parts.[1] Compared to electromechanical HDDs, SSDs are typically less susceptible to physical shock, silent, and have lower access time and latency. SSDs use the same interface as hard disk drives, thus easily replacing them in most applications.
And here is the system I ordered:
Qty. Product Description
Motherboard
1 ASUS P8P67 DELUXE (REV 3.0) ATX Intel Motherboard (You can also go with the new Z68 chipset, which allows SSD caching and video transcoding)
Item #:N82E16813131701
$229.99
CPU Cooler
1 Antec Kuhler H2O 620 Liquid Cooling System
Item #:N82E16835209049
$79.99
Case
1 SILVERSTONE RAVEN RV02-BW Matte black Computer Case With Side Panel Window
Item #:N82E16811163154
$179.99
SSD
1 OCZ Agility 3 AGT3-25SAT3-120G 2.5" MLC Internal Solid State Drive (SSD)
Item #:N82E16820227726
$234.99
Blu-Ray + DVD Burner
1 SAMSUNG Black Blu-ray Drive SATA Model SH-B123L/RSBP LightScribe Support (Retail)
Item #:N82E16827151232
$64.99
Video Card
1 EVGA SuperClocked GeForce GTX 570 (Fermi) HD 012-P3-1573-AR Video Card (Lifetime Warranty)
Item #:N82E16814130622
$354.99 (You can also go with the AMD 6970, performance is a few hairs different as is price, both cards support more than 1 monitor)
OS
Microsoft Windows 7 Home Premium SP1 64-bit English 1-Pack
Item #:N82E16832116986
$99.99
HD + PSU Combo
1 SAMSUNG Spinpoint F3 1TB 3.5" SATA 3.0Gb/s Internal Hard Drive -Bare Drive
Item #: N82E16822152185
1 CORSAIR HX Series CMPSU-850HX 850W Power Supply
Item #: N82E16817139011
$204.98
RAM + CPU Combo
1 G.SKILL Ripjaws X Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory
Item #: N82E16820231445
1 Intel Core i7-2600K 3.4GHz LGA 1155 95W Quad-Core Desktop Processor
Item #: N82E16819115070
$384.98
Total: $1,834.89 (These are all Newegg prices)
For the monitor I would go with:
ASUS VH242H Black 23.6" 5ms HDMI Full 1080P Widescreen LCD Monitor 300 cd/m2 1000:1 (ASCR 20000:1) Built in Speakers
$179.99
Grand Total: $2014.88
Hope that helps giving you a starting point...