I'm building a new Desktop PC with a $700 budget as well, pretty much waiting for Newegg's Black Friday sale to get the CPU/MoBo. If you live in the US you can try Newegg.com for barebones/components or Computerlx.com for custom systems (3 year warranty + Lifetime Tech Support).
CPU
Best bang-for-your-buck CPU right now is the
Intel i5 SandyBridge 3.3Ghz 2500k -- I've used both AMD and Intel and they're both great. Intel is still better IMO, their factory heatsinks/fans are solid, compared to some of the factory AMD fans.
Motherboard
You would need a Socket 1155 Motherboard to go with that CPU - if you want upgrade-ability look for USB 3.0, Sata 6gb, PCie 3.0 -- If you have an old IDE/PATA drive you want to keep make sure the MoBo has an IDE connector.
Brand Compasisson:
-
ASUS - Solid unless you get a DOA . Customer service: Terrible.
-
ASRock - Even better than ASUS (ASUS is a parent company of ASRock) Customer service: None.
-EVGA - Reliable, expensive components, a bit outdated. Best customer service in the US.
Video Card (GPU)
- Best video card under $300 is still the nVidia GeForce 560 ti (costs $250 @ newegg)
- under $200 = nVidia GeForce 460
- under $150 = nVidia GeForce 440 or 450
** You can see them all
here
Best brand = EVGA
** Most of these new video cards are HUGE ( 8"-10" ) so make sure your case has enough space for the wires + video card.
Power Supply:
If you buy a 560 ti or a 460+ you might want to get a PSU with at least 600w. Corsair 650w should be more than enough. Make sure the PSU has the power connectors for whatever video card you choose: some need 2, some of them only need 1.
RAM:
4-8 GB
Good brands: G.Skill, Corsair, Patriot