yes and no - with those specific parts I would spend more for the cards and less on the motherboard/cpu (I'm in Australia).
However overall, if you are mining BTC and the computer will never be used for anything else (such as gaming) and you want to get cheaper consider:
dropping the CPU to a
- dual core and slowest as you can realistically afford (an AM2+)
dropping the motherboard to a am2+ models, but also with 4 pcie (as many pxiex16 sized as possible, otherwise you have to use risers or an additional power source which may introduce some burnout unless you have good quality cables - I Think its 25watts for x1, and 75watts for a x16. Also, as you have 4 cards (even though they are 7850s) you might end up running to much voltage through the motherboard (although the Ultra Durable 3 boards seem good so far for myself, but I don't run 4 cards on one motherboard)
Optinoally, just replace multiple 7850s with an 7870 (pictairn) or bump up to a 7950. Atleast over here, for mhash and power comsumption, the 7870 and the 7950 are the most efficient, for mhash to base price, and mhash to power usage. I was considering 7850s aswell, but hte 7870 blew it out of the water, as did a 7950 (which uses the newer tahiti chips). Don't forget that all because you can cram 4 cards into one rig without risers, doesn't necessarily make it that good if you can get 2 or 3 other cards to do the same job, with better better thermal and power efficiencies and a similar base cost. It also means you can then sell them off later - or atleast use them for an overkill crossfire system jsut for gaming and running 16 monitors off for some reason. 
Basically, as you are using GPU mining, you really want a system that is:
cost effective vs total mhash
won't be a peice of junk in the corner of your garage if you stop mining any time soon
That leads to for the first point, you want the overall mhash to be high and the cost and power consumption to be low (The later is a huge factor over here where we pay around $0.30/kw/hour in USD.)
For the second point, odds are that if you ever do game on an AMD card, you won't use more than 2 cards in Crossfire. Or even one card. So you have a bunch of spare cards - so if you don't use them you want to sell em.
Out of the box, a stock 7850 will reap around 260mhash/s. a 7870 will do around 390mhash/s 7950 wil do around 430 mhash/s
When overclocked, to 1100mhz for the 7850, 7950 and 1200 for the 7870, you will get: 7850=340m#/s, 7870=460m#/s, and 7950=590m#/s
You could push it higher, but thats what I run and they stay pretty cool (even though I run them all in the same case - I get 64*C, except on the top card which gets all the heat from the lower cards which runs at 68*C. I haven't been bothered to split them out of the case yet - but when stand alone they run at around 50 to 55*C.
So basically for a 4 gpu system with 7850s, you pull in 1350 m#/s. Thats around 10% faster than running 2 7950s. It will also consume, around 600watts give or take, where 2 7950s will do around 400. (3 7870s will probably draw around 450). Due to higher power going through them you will also generate more temperature (think of it as a 600watt heater compared to a 400watt heater).
The GPUs that you are looking at have 2 fans each only. If you pick up a 3 fan 7950 or 7870 (which atleast the Gigabyte ones come with), you would most usually push through more air - which is important if you stick them close together - as that motherboard can take 3 double width GPUs you may atleast do that for 3 of the cards and a riser on the 4th. Stick two on there and you shold be fine, and the other two on risers. They will still get hot though.
Anyways, so your pulling more power and generating more heat which you have to disipate, or just use more risers and a bigger box.
So atleast compare the cost of 3 7870 pictairns, or 2 7950 tahitis or above for a replacement to those four 7850s
Additionally, 2 7950s on that board at the furthest gap would let you mount them with 3 rails space between them without risers, all running off the power from the motherboard and one Powersupply, with space to sapre if you ever want to stick on 2 more cards (assuming you are using windows where it appears to be a four card limit). Also, if you let the motherboard lay flat (desktop style!) your heat would disipate away from the rig, as opposed to the next card in tower orienation. 2 cards also means less dummy plugs or cables going to your monitor if you go that path.
And to boot, your power supply would last longer, or optinoally, you could drop down to an 900watt bronze, as you would still be around the 50% mark for long life efficiency. Or stay at that 1200watt giving room to spare in the future.
As for the ram, I'd personally get 4gb if you plan on using windows. It just runs better if you ever have to actually use it and its pretty cheap.
I hope this helps a bit.
But otherwise, good price hunting!
