Hello and welcome
I have some options for you to consider:
1. The 850W PSU is not overkill. It is best to spec you PSU to run at 50% capacity if possible. I would recommend that you go with the Corsair HX850 which regularly goes on sale for $140 - $150. It has several advantages:
* It is modular - you won't have cable mess in your case
* It will run at 92% capacity at 50% capacity which is nearly platinum performance
* It will run very quiet and cool
* It comes with a 7 year warranty from a company with legendary customer service
* It comes with an abundance of cables (more than you will need)
* It will easily power two cards, it will put out over 1000W and remain stable
There is a rash of posters that are stuck on recommending the cheapest, lowest wattage PSU that will run your computer. This is a mistake. ANY PSU will run better at 30% - 60% than at 70% - 100% capacity. It is just a simple fact.
There is also the "200W rule." That is, specify your PSU at 200W over max capacity. This is somewhat acceptable but it is inferior to specing your PSU at 50% max capacity. Your PSU will simply run much more efficiently at lower capacity.
BTW I own the HX 850 and so I speak from experience. It is simply a superb PSU. I run one ATI XFX 2GB 6950 and I have never even hear the fan turn on (it may have but it is silent if it is running).
2. Go with a cheap 120GB SSD like the Kingston Hyper X Extreme or the Corsair GT Force (emphasize the GT). These are usually at $200 or less and they are very fast. I own the Corsair GT, it is a very good SSD. The Mushkin Chronos Deluxe is probably the fastest 120GB SSD but it is also expensive and it is not that much faster than the GT or the Extreme. Avoid the OCZ Agility and Solid if possible as they have a cheaper Nand flash memory.
The SSD will speed up your whole system and make everything much faster. It is a joy to use a system with the operating system on the SSD
3. The 1 GB 6950 or the 560Ti are both very good video cards for the money. They are both recommended by Tom's Hardware. The brand is not overly important. I like XFX for their lifetime warranty. The 560Ti not the 560 is the way to go if you are going to buy Nvidia. Check to see if you can get a 2GB 6950 on sale too, sometimes they are really cheap
4. A great midtower cases are the Corsair Carbide 400 / 500, the Corsair 650D. The Silverstone Raven is another very good case - but it is very big. The Cooler Master 922 is worth a look. I own the 650D and I think it is an amazing case. It is pretty conservative compared to many gaming cases. I am not sure which cases are "easiest to move from house to house," some of the "lan" cases are pretty mediocre. The Cooler Master 690 II is also a cheap midtower that is very good.
5. The Samsung Spinpoint F3 1 TB and the Seagate Barracuda 1TB drives get recommended a lot for good reason; they are very good drives and they can be found on sale for around $50 on Newegg. However it appears that HDD prices are going up lately.
6. Get a motherboard that is Ivy Bridge compatible and PCI-E 3.0 compliant. They will be ready for the next generation GPUs as well as the new Ivy Bridge CPUs. So if you want to upgrade, you will not need to change out your motherboard. The AsRock Extreme 4 Gen 3 is in your price range and is an excellent board.
7. Since you are going to overclock I would recommend the Noctua NH-D14 as a CPU cooler. It is a legend among overclockers and is perhaps the best overall air cooler. The Hyper 212 Evo recommended earlier is a cheaper alternative and it is a capable cooler but with a $1200 budget you should be able to afford the D-14.
8. The 2500K is the perfect CPU for your build. Check Amazon for pricing because they are frequently cheaper than Newegg and they have a better return policy.
Good luck with your build