Here was my entry in the small office category:
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Homebuilt system Masterx . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x . x .
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Geofelt small office pc.
I recently built essentially this same pc for my son's mother-in-law who runs a small business.
Here are the salient reasons:
1. I used a large SSD. It is the critical component for performance. 240gb is sufficient for normal storage.
If necessary, a hard drive can be added for bulk storage.
2. Dual core is sufficient, the i3-3220 is actually more than you need but fits within 64 cents of the budget.
3. The lian li Q07 is a top quality case that is small enough to fit on a desktop without consuming excessive space.
4. I have used ECS before, and found them to be of decent quality. Partpicker warns of possible incompatibility with ivy bridge, but any new shipment since January will have the needed bios already installed.
This pc is capable of using dual monitors which I highly recommend.
PCPartPicker part list / Price breakdown by merchant / Benchmarks
CPU: Intel Core i3-3220 3.3GHz Dual-Core ProcessorP ($118.99 @ Amazon)
Motherboard: ECS H61H2-I3 (v1.0) Mini ITX LGA1155 MotherboardP ($49.99 @ Amazon)
Memory: Patriot Viper 3 8GB (2 x 4GB) DDR3-1600 MemoryP ($54.99 @ Newegg)
Storage: Samsung 840 Series 250GB 2.5" Solid State DiskP ($174.70 @ Amazon)
Case: Lian-Li PC-Q07 Mini ITX Tower CaseP ($64.99 @ Newegg)
Power Supply: Corsair Builder 430W 80 PLUS Certified ATX12V Power SupplyP ($19.99 @ Newegg)
Optical Drive: Asus DRW-24B1ST/BLK/B/AS DVD/CD WriterP ($16.99 @ Newegg)
Total: $500.64
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-08-07 11:12 EDT-0400)
You can read the rest of the entries here:
Some are good, some are bad:
http://www.tomshardware.com/forum/id-1757446/bestconfigs-intel-based-office.html?xtor=EPR-8809#11365673
Onboard HD 7.1 sound is quite good. Only if you are attaching a high end set of speakers might a discrete sound card be a bit better.
The cpu is relatively unimportant for your needs; a decent duo is fine.
To mi mind, the most rewarding component will be a ssd large enough for your files. It will make everything feel so much quicker.
And... see if you can't add a second identical monitor; It is wonderful for productivity.