While it's true that GPU prices are an important factor, it doesn't matter how great of a deal you get if you can't get the card to produce good results. What are the hash rates on your other 5830s? I've found that the 5830's are about the best cards that are presently available which provide the most "bang for your buck". For example, if you can press them above 300Mh/s, which is a reasonable rate for that card, you're already getting over 2Mhash per dollar spent on the 5830 card as long as you paid under $150 for it! A quick search on Newegg shows a
Sapphire 5830 for $129.99, which is well within these parameters. Not a bad ROI at all!
The post before mine mentions a 5870 card for $189, which would be a good deal as well, however I can't seem to find a brand new 5870 listed for sale anywhere on the web, let alone for a price anywhere close to that. The Radeon 58xx cards seem to be harder and harder to come by anymore. It wouldn't surprise me to see the Sapphire 5830 from Newegg disappear soon as well. At $130, it's not a bad deal at all, but again it all depends on what hash rates you can get from your cards. Good luck!
The problem is your thinking about only the card. The card alone can get over 2 megahash but when you add the cost of an entire server the best you can expect to see is around 1.58 megahash per dollar.
I'm getting around 440MH/s out of mine. I can't remember where I saw it, but there was a chart comparing MH/s, cost, and power consumption and the 5870 is THE card to get for MH/s vs. Power used.
If you add up the costs for a server (bare minimum)
cpu- $30
ram- $10
psu- $150
mobo- $140 (if you know cheeper then this with 4 pci express then let me know)
hd- $15
PCIe x16 Extender Cable $12*4
5870 $190x4
= $1,153
(440x4)/1153=1.53 megahash per dollar(full server cost)
with 5830
5830 $110x4
= $833
(330x4)/833=1.58
So as long as you go the absolute cheapest route 5830 rig at 110 is better but not much so. If you add a case or more to the cost at all the 5870 at 190 will probably overcome the 5830.
Using the calculations above and adding 80 dollars to the 5830 machine (4x the diference of the 110 vs 130) you get a ratio of 1.44 which is about .1 less megahash then the 5870 and .15 less then the 5830 at 110.
This is the very important part if you invest more money into the server then this the gaps between the ratios get even larger.
I recognize that these cards are not for sale at the moment and the 5830 at 130 is probably the best deal at the moment. That doesn't mean you should buy it. If I am going to invest in more hardware its going to most definitely be a better card then the 5830 at 130.
