Is $500 your full budget for a complete case/power supply/OS/etc..?
Any reuse from an old system? What are you figuring your costs are for the common items (case/power/optical drives)? The 870 board doesn't have onboard video, so you'll need some GPU to get the system running. The 890GX could at least start out with the integrated GPU until you add the discrete card. The 2GB RAM would get 'wasted' when you upgrade to your 16GB 'destination'. If you can't get 4GB module(s) to end up in a 4x4GB=16GB configuration... there will be some 'repurchasing'. Unless you're storing DVD rips, you probably can get by with the 250GB drive (although the 500GB probably isn't more than $20 extra).
Look out that avoiding spending some money now doesn't cost you more in the long run. Whereabouts are you located and what sort of local stores are available to you? Just curious what the options are like.
I would go with an x4 955 processor over the 1055T. What exactly are you going to be doing with the PC? If it's gaming then go with the x4 955. If you're using heavily threaded apps then go with the 1055T.
Personally, I bought the x2 555 and unlocked it, but that's not a guarantee. You may get stuck with an x2 555 that is actually only 2 good cores. Mine is sitting pretty at 3.7ghz and 4 cores, but looking back I should've gone with the 955 and not taken the risk =P
Go with the AMD x4 955 b.e. over the 1055t like pelov said or possibly the x4 965. You will get better performance in most applications with a 3.2ghz quad core or higher then you will with the x6. overclocking and unlocking cores causes instability and heat issues. The 555 is not a good choice. Definitely go with an 8 series mb chipset 870 and 890fx don't have on board video. 880 and 890gx have on board video. You don't need an 890fx unless you want to run multiple gpu's and with your budget, you really can't afford it. use the extra money you save between the 1055t and the 955 and get 4gb ram instead of 2gb.