Which one would be better for a work/business computer, where I would install an accounting system?
AMD Phenom II x4 810 2.6 GHZ 6GB RAM 1TB $600
AMD Athlon II 630 Quad Core 2.8 GHZ 4GB RAM 750 GB $560
The extra $40 per computer gets you:
CPU - only slightly faster than athlon II quad core. They are basically the same thing except for on chip cache. Has an impact in games, less so for other tasks.
RAM - 2 gbs more. Current cost of 1 gig is around $20
HDD - difference of 250 gb, you likely wont fill half of either drive, so this category doesn't matter.
Either CPU will work fine, I can't say if there will be a strong impact of the cache on the phenom producing better performance because it really depends on the software. It would be at most 10% difference. At $40 difference, I would just get the phenom build, but if you are really tight on budget, skip the extra ram and HDD space and get the cheaper athlon build.