You won't be able to get a motherboard, i7 860, and memory for $300. The processor alone is $220, the cheapest motherboards are $100, and 4GB DDR3 is around $70.
If you're very much into overclocking, OR if you plan to run a SLI / Crossfire setup at any point, get the i7 920 and X58. If you'd rather stay at stock speeds and only plan to use a single graphics card, i5/i7 and P55 is a good choice. Dual vs. Triple channel memory makes virtually no difference in real world performance; the only real advantage (in my opinion) is the fact that you can load your motherboard with a greater number of cheap DIMMs. 3x or 6x 2GB DIMMs vs 2x or 4x 2GB.