what parts do you have already that you can salvage from your old computer? I.E. your tower, PSU ect.... and since you say you dont have a budget i'm assuming money isnt an issue for you.
I'd recommend going future proof if your going to get yourself a new PSU and get over 700w. I got myself a 950w PSU
Next are you AMD or Intel. There is a big decision point. From what I hear AMD is coming out with a new processor soon (as philsabot has stated) that is going to top intel's gear. Only thing if you wait for that is that its going to be super expensive, not too many compatible mobo's ect... Go with at least an intel Core i7 or an AMD Phenom II x4 BE with no less than a 3.2GHZ clock speed.
Also your going to want to buy a new heatsink because stock ones are badddd.... so just find one you like and get it. Make sure to get some thermal paste too. The stuff they give you with the heat syncs arent as good as they should be.
Next is your mobo. What do you want in your motherboard. I'd suggest finding one that has at least 2 pci express 2.0 for the option to run crossfire / SLI. Note that depending on if you go with amd or intel will limit your ability to run crossfire / SLI. Intel boards support SLI and AMD don't. Find a board that is capable of holding at least 16gb of ram being that the ram standards are beginning to increase, (at least among the gaming community). And most of all make sure the board is compatible with the CPU your getting. (Check newegg for combo deals too.... They usually have some pretty good ones. Although be weary that newegg has been known to make combo deals for parts that arn't compatible so always double check). Also note while looking for a mobo, check the form factor. I like to go full ATX simply because it allows for more space for larger cards and cable management..... Note the cable management. Good circulation means healthy computer.
Next is your GPU. If your not looking to make BF3 as pretty as it can get, just go with a GTX460. I personally don't like ATI as much and nvidia cards tend to out perform ati cards. To find a good card to your liking go to
www.tomshardware.com and look at benchmarks for all the different cards out right now. But if you want to go big get a 570 or 580. The 580 is actually what the developers of BF3 used on their computer when they recorded the teaser trailers.
Next is your ram. Check the types of ram your motherboard supports and pick a clock speed. not really much to say here... except avoid a couple brands. I'll recommend Corsair, G.Skill, OCZ, Crucial, and PNY. They are pretty reliable. and I put them in order of which ones I would buy first if they were all identical.
Next grab yourself a SSD. I run one right now and my computer is LIGHTNING FAST! mainly because of the SSD... lol I have a 64GB crucial SSD that just runs my OS but if you want to throw your games on there i'd recommend getting one that is a little larger than that but they cost a pretty penny so dont over do it with the HDD alone. Next i'd grab another 2tb or so just for all of my personal stuff... Its nice to have your OS and your other stuff on seperate drives, simply because computers like to screw you over and crash and lose all of your data... if that happens your stuff is safely stashed away on your other hdd.
as for the tower just make sure it is the same form factor as your mobo.
and grab a large monitor, or 2. Note that if you want to run 2 cards with 3 monitors, (eyefinity) you need 3 monitors of the same size. if not you'll have to match the resolution of the smallest monitor and it will look bad..
well thats really it. without a budget, preferences or more specifics i cant really help you more than that. Just use this as a guild line and tell us what your looking for. Hope i could be of assistance to you.
PS: check peoples signatures, they tend to brag about what they got in their computer. see somebody with something you like. ask them how it is.