With the increased budget, you should definitely get yourself the AMD Radeon HD 7970. The 3GB of VRAM on the card itself won't do much unless you're cranking the anti-aliasing, so it does come in handy for some titles.
384-bit versus 256-bit is just the internal bandwidth - think of it as the higher those numbers, the more numbers the GPU can crunch inside of itself. The higher the number it can crunch, the more performance it provides.
As for overclocking, some of the GPUs come with a pre-overclocked state. Some of them are advertised with "Boost", which is what that means - it has been overclocked for you.
CrossFireX is multi-GPU action, more than one Radeon working together to increase performance. CrossFireX (CFX) can work with up to 4 GPUs, and this type of setup is only for those with super high-res monitor setups, or multi-monitor configurations.
I would look at getting Sapphire's Radeon HD 7970 3GB OC with Boost - this card is $409.99 from Newegg.