It will help if you state what the main purpose is for the computer and any other uses.
Also, it helps if you let people know if you want to overclock your Ivy bridge CPU when you get it. Also do you plan to use an SLI system (two 580s in SLI)?
Finally, though your budget is fairly clear but do you have a minimum and maximum budget for this build?
1. The 2500 or 2500K is your best gaming chip, but since you don't post a budget, I am not sure if this meets your needs. If you plan to still use this computer in some manner (i.e. give it to your little brother) then it my be worth the extra $75 to move up to the 2500K which is a very fast CPU.
I am not familiar with the 2100 performance for gaming and so I will let others comment on that.
2. The 1155 board you post will not overclock and I have a feeling that you will need a higher level board for Ivy Bridge. There are people more qualified than me to comment on this though. Most gamers now are overclocking their Sandy Bridge CPUs.
If gaming speed is important than probably a cheaper P67 mobo with the 2500K would be the best choice. But if your on a tight budget this may not be your best option. Several people who post are very knowledgeable about putting together low budget builds.
Also check Bit-Tech's and Tom Hardware's low budget builds for some valuable information.
3. 4GB or RAM is fine to get you by. You may want to move up to DDR3 1600 or DDR 1866 for better performance
4. I used to own the ATI 5850 and it is a great card. I paid over $300 for mine, darn it ha, ha. It will work for a gaming rig to get you by. Overall it is an excellent card with surprising processing power. The replacement card that is sort of the equivalent of the 5850 today is the ATI 6950.
If gaming is your goal and you have the budget, you may want to consider the 1 GB or 2GB 6950 which you can get for around $225. When you build your Ivy bridge rig you will be able to use this card and go crossfire for a very effective gaming system.
5. If you are going to build a bigger Ivy Bridge system from the parts of this computer, I would suggest buying a PSU that is big enough to power your future computer. For a future crossfire system the Corsair 850 HX is an excellent choice. For a single GPU system you could go with the Corsair 750 HX or the new Corsair 750 TX. Note that a 650W PSU will power a one GPU system and Corsairs 650 HX is a very good PSU. I think Corsair makes a 650 TX also which will be comparable in cost to the unit you have listed.
If you are going to use this computer after your new build, then a 550W - 650W is more than sufficient. Corsair units frequently go an sale and you can get great deals. They also have the best warranties in the business and their PSUs are highly rated.
If you are going to two 580s in SLI for your future system then I would recommend that you look closely at the Corsair AX 1200. It costs around $260 and it will power your SLI 580 system more quietly, more coolly and more efficiently than an 850 W unit will. It is also better than the Corsair 1000 HX unit which was not as efficient as the AX 1200.
Keep in mind that you will not use the full capacity of the AX 1200 in an SLI configuration but that shouldn't be your goal. You want to keep the unit at about 25% - 50% capacity where it is most efficient.
If budget is an issue the 850 HX will run two 580s, it just won't be as quiet and efficient as the AX 1200. For a single 580 I would recommend the Corsair HX 850
Finally, for the system you list the 550W is enough to power it, if your PSU is efficient. Check reviews for efficiency.
6. A lot of people use the Asus optical drive you list and so I am sure it must be good (or it wouldn't get used).
Sorry I forgot to mention; I did see your note on the GTX 580 and it is a great card. However two 6950s in crossfire outperform the GTX 580 and the cost is about the same. I wanted to let you know that option is available for the same cost.
The 580 is an amazing card and is a great choice also. Check out Tom's Hardware's Best GPUs for the money for May. It will give you some valuable advice on purchasing your GPUs