I wanted a 550 ti, but the 6850 performs WAY better for the same price. What would you recommend for under $150?
Ah, if price is a concern, the 6850 is hands down the best bargain on the market. You can even dual-link some time in future hardware upgrades it if you get a motherboard with multiple PCI slots. I have no experience with dual-linking, but it's worth looking into. Two mediocre cards are often better (And, more importantly in your case, cheaper) than one beefy card. Just buy the next one once you've got a few bucks to spare.
If you don't mind pushing the buck, though, the 560 ti is a slight step up in terms of power - But the main reason you'd want it is to avoid compatibility issues that AMD sometimes has. It's a hefty $50 above your GPU budget for most decent models, so it may not be the best choice at the moment. Don't touch the 550 ti - It sucks compared to the 6850, even with the Nvidia label.
Also, how high do you think I could overclock the 2500k with that copper CPU heatsink I linked?
It depends. Looking through the reviews it sounds like you'll be able to up it a few notches, but you'll want to see what the benchmark temperature is without any OC'ing at all. Just don't let the temperature get above 78C, the recommended maximum temperature for Intel chips, and you'll be good.
You may consider the
Enermax ETS-T40 Cooler, which not only costs less than the one you're looking at ($10 cheaper), it's also quieter, and rates in second place in
this late 2012 round-up, failing only to beat a water-cooled system that runs about $144 right now. It is larger, though, so be sure that it'll fit in with the motherboard and the case. I'm also not finding any comparable benchmarks for the one you're looking at, but I'm kind of put off by the Zalman's age (It was released back in 2005, from what I can tell). It's worth doing some research.
Of course, this is another area in which you can easily upgrade as you need to, but coolers are pieces of hardware you can easily transfer to newer, better systems since they have a long life and new models tend to improve at a slower rate than other bits of hardware. They're good investments if you plan to cannibalize old computers for new ones in the future.
No matter what you decide on, I highly recommend buying a separate tube of thermal paste. The paste that typically comes with coolers is notoriously crummy, and dropping ten bucks for something that does the job better and longer is definitely worth it. On NewEgg, everyone seems to love the
Arctic Silver. Can't really argue with the masses when the core temperature drops by 10C in some cases.