Some points:
First, I missed the fact that you are doing a lot of upgrading:
1) do NOT get the GTX 4xx series. They run extremely hot and noisy. Go ahead and Google that. If you're getting a new NVidia card you want a GTX 5xx series. (two GTX 470's would be a nightmare for noise)
2) you need to be careful getting two graphics cards with your 750W PSU. The amount of Amperage (current) available is important. Power Supplies vary a LOT in quality and how they distribute the power.
3) Most games will run very will with a single GTX 560Ti. Your PSU may handle 2xGTX 560Ti but I wouldn't go above that. (in fact, power aside you may run into some memory bottlenecks but I'm not certain)
4) SLI is usually better than Crossfire so beware if they say a single HD6870 is "better" than a similar NVidia card if going SLI.
5) benchmarks:
http://www.guru3d.com/article/geforce-gtx-560-ti-sli-review/4
Summary:
- check PSU specs carefully
- recommend a single GTX 560Ti (2xGTX 560Ti at most if PSU can handle it)
- noise is an issue for SLI (forget the GTX 4xx series completely)
- many games are maxed out by the new CPU and a single GTX 560Ti anyway
- I went with a single Graphics card due to the heat and noise added by a second, NOT for lack of money.
- *If that single GTX 560Ti is not enough buy a second one but wait to see if it's good enough. I'd recommend keeping it for two years then getting a better single card to replace it.