Post above is pretty good if willing to make compromises (you won't get pro quality 3D work out of a $700 rig, so compromises will have to be made). i5 is a good processor to build from as it's mid-tier. Not the best, but not the worst and you save cash. Most motherboards that can accept i5s can also accept i7s, so you can upgrade later when you have the money. 8GB of DDR-3 memory is the standard now.
Pro tips to always remember about build your own computers:
1. It's important to get the best motherboard you can afford for just that reason -- to upgrade. Make sure to goto the manufactuer's website for the particular motherboard and look up 2 things -- what memory it can use (especially it's clock speed and timings) and what video card. That will save you a ton of woe. I'm finding AsRock motherboards are pretty darn good for the price (AsRock is a division of Asus, so it's not some cheap knockoff company). It's what I have powering 2 $2,000+ rigs, 24/7.
2. Get the best PSU you can afford -- never ever skimp on a PSU, cheap PSUs can
literally blow up (especially under the workload of a heavy gamer) and destroy everything. You want at least 600 watts and 42A for the 12v rails (as that's what the newer video cards require to operate -- so look up the technical info to make sure [Newegg will list it]). More 5v rails = more fan and accessory options too. I personally stick with Seasonic PSUs, simply because they underrate their power (600 watts really is more like 680...extra cold start power) and they last. Still have a Seasonic 600watt from 5 years ago. Rock solid rails (does not go below or above 12/5/3.3 volts).
3. Memory: Buy the 2 or 4 memory stick kits -- matched memory is more stable. Try to use only 2 sticks (4 sticks causes power issues in a lot of motherboards, save the headaches). Also, lower the numbers (not higher) is better. So if you see 2 sticks and one is 9-9-9-9 and 8-9-8-8 get the latter. Every clock cycle that is lower = faster output. Stick with tried and true brands for quality assurance -- Corsair; Crucial; G.Skill; Mushkin and Patriot. Have a very nice set of G.Skill Ripjaw X 2133mhz and find that brand trouble free (all memory higher than 1066mhz is overclocked, just so you know. If you put 2133mhz memory in a motherboard if the SPD isn't recognized it'll default to 800mhz, which is default speed of DDR3. Saying this as some get confused..."I bought 2133mhz but it's only showing 800!!!"). Newer motherboards and memory now do all the timings for you, so you don't need to mess with them. Just leave those timings alone until you know how to overclock (one bad move there, bye bye memory, or worse motherboard).
4. Video card: Player choice. If you want to do Folding or SETI, no doubt use Nvidia. AMD video cards is what I prefer due to longevity (don't like replacing $300+ every year because a Nvidia card dies just at the end of it's warranty -- EVGA/XFX you name it quality brands, die, kaput, killswitch material )
5. HDD: If you don't store important stuff on your computer (or backup often) RAID-0 2 hard drives (default settings are fine). Faster output. It's gamer preferred (let alone if you want to move video faster). SATA 6.0 drives with 64KB directories are preferred (larger directory = faster lookups). The Western Digital Black 1TB series tend to be the overall favorite (good drives, good warranty...as they can and do fail).
One MAJOR suggestion: buy the HDD from Amazon, not Newegg. Newegg puts them lose in a box full of air pockets and/or packing peanuts...stuff that ruins HDD from physical damage. Amazon ships them in a easy to open form fitting cardboard OEM box with anti-shock ends (Amazon is great with packaging, so much so I don't use Newegg anymore, the price difference is not worth having to go through returns because a UPS gorilla dropped your box).
Yes, I love computer hardware.