One way to save $ on systems is to integrate the video and sound on board the motherboard.
Well that works for basic computer tasks: office work, typing letters, internet browsing, email, etc.
But for gaming or other performance-oriented computing you'll actually spend more in the end; after adding a real graphic card, real sound card, new power supply, faster RAM, new motherboard, etc. it would have been cheaper to buy a decent machine to begin with.
----------------
To the OP, I can't comment on those particular graphic cards you've asked about - there are new models released every month and I only build once every couple of years - I couldn't possible keep track of each new product.
But here are a couple of things to consider when shopping for / building a high-performance gaming machine:
1) Case. Good air flow. Room for expanding. Ease of access.
2) Power supply. I'd be looking for a 700watt supply for modern graphics, hard drives, accessories, etc. 1000watt supplies are now available, but probably overkill for most folks.
3) Sound. A good surround sound system (sound card + speakers) that won't distort at high volumes.
4) Graphic card. Self explanatory.
5) Monitor.
Here is an example of the system requirements printed on game packaging. This was a game, randomly chosen, that was released last year - current and future games will undoubtedly have higher requirements.
Recommended Requirements
CPU - Intel Core 2 Duo processor
System RAM - 2GB
Video card - DX9 - Direct X 9.0c compliant video card with 512MB RAM (NVIDIA GeForce 7900 GT or better) / DX10 - NVIDIA GeForce 8600 or better
Sound Card - Sound Blaster X-Fi series (Optimized for use with Creative Labs EAX ADVANCED HD 4.0 or EAX ADVANCED HD 5.0 compatible sound cards)
I hope that gives you some ideas. Happy shopping!