I'm asking this because is hard to calculate future profits.  If I mine in bitparking I can trade all merged coins for bitcoins in the market, well except for groupcoins, can't find a market for it.  Multipool doesn't have merged mining, but it always mines the most profitable sha256 coin.  Anyone that mines in these pools can give me an opinion about which one could give me the best profit?

This applies to any cryptocurrency, let's say I build my machines and start mining.  and I built up to about 1000 coins.  Decided it is a good time or I need to sell  my coins.  Where is the right and best place where I can go and sell those coins? you can specify the kind of coins if necessary.

http://www.dell.com/downloads/global/products/pedge/en/2850_specs.pdf

I found this add on craigslist selling this servers for about $125 with 2 intel xeon 2.8 ghz, 2 36gb ultra 320 15k hard drive, and 4gb ram.  There are little more expensive ones if more ram and hard drive space needed.
So my questions are if is it possible to add two 7950 radeon cards to these servers? and is it worth it to even try?

Thanks to everyone that has helped me. I decided to go with the radeon 7950 cards instead of the 7970.  It saves me $200 and with a difference of  0.02Mh/s/$ lower. I made my calculations with the 7970's I can hash up to 1400Mh/s if oc, with the whole system price of $1100 gives me 1.29Mh/s/$ spent. With the 7950's I can hash up to 1200Mh/s oc, with a price of about $930 for the whole system it gives me  1.27Mh/s/$ spent. So it's not a big difference it saves about $200 dollar.  I will use ubuntu 12.04 LTS on a 16 GB thumb drive I already own.

I changed my mind, I will use milk crates, it looks pretty easy to do after reading some guides. Also swiched from msi 970 mobo, to asrock 970 and switched saphire vapor x to xfx.

http://pcpartpicker.com/user/thehulkk/saved/1wd1

CPU: AMD Sempron 145 2.8GHz Single-Core Processor  ($35.98 @ Outlet PC)
Motherboard: ASRock 970 Extreme3 ATX  AM3+ Motherboard  ($74.99 @ Newegg)
Memory: Patriot Signature 4GB (1 x 4GB) DDR3-1333 Memory  ($28.87 @ Amazon)
Video Card: XFX Radeon HD 7950 3GB Video Card (2-Way CrossFire)  ($263.98 @ Newegg)
Video Card: XFX Radeon HD 7950 3GB Video Card (2-Way CrossFire)  ($263.98 @ Newegg)
Wireless Network Adapter: Encore ENUWI-1XN42 802.11b/g/n USB 2.0 Wi-Fi Adapter  ($9.98 @ Outlet PC)
Power Supply: Cooler Master Silent Pro M2 720W 80 PLUS Bronze Certified ATX12V Power Supply  ($74.99 @ Newegg)
Total: $752.77

Hello, I'm fairly new mining, I've been mining for about a week with my laptop that does 24Mh/s, and it feels that it's going no where so I found this two gaming rigs on Craigslist that looks very promising for mining, I hope someone with moreknowledge on hardware can give me better advice, I feel like the first one can do better for mining but I could be wrong. Thanks ahead of time.
1-
Custom Gaming Computer - $940 OBO

CPU: AMD Phenom II 955 x4 @ 4GHz stock voltage tested stable in prime95 water cooled
GPU: XFX 7950 Double Dissipation with lifetime warranty
DRAM: Two 8GB Crucial Ballistix Elite Rated for 1866 at 9-9-9-27 1.5v currently tested stable in prime95 at 1600 7-7-7-21
Storage: Sandisk 64GB SSD 490MB/s read
Main board: MSI GD-65 890FX with two full 16x PCI Express 2.0 Slots
PSU: XFX Pro 550w 80 plus efficiency
Case: Black Antec Lanboy Air
Disk: Lite-On DVD burner 24x read

2-800
CPU: Intel Core i5-2500 3.3GHz Quad-Core Processor
CPU Cooler: Cooler Master Hyper 212 Plus 76.8 CFM Sleeve Bearing CPU Cooler
Motherboard: MSI H67A-G43 (B3) ATX  LGA1155 Motherboard
Memory: Kingston 8GB (2 x 4GB) DDR3-1600 Memory  
Storage: Seagate Momentus XT 500GB 2.5" 7200RPM Internal Hard Drive  
Storage: Western Digital Caviar Black 1TB 3.5" 7200RPM Internal Hard Drive
Video Card: Sapphire Radeon HD 6850 1GB Video Card (2-Way CrossFire)
Video Card: Sapphire Radeon HD 6850 1GB Video Card (2-Way CrossFire)
Sound Card: Creative Labs Recon3D PCIe 24-bit 96 KHz Sound Card
Case: Rosewill RANGER ATX Mid Tower Case
Case Fan: Cooler Master SickleFlow 69.7 CFM 120mm  Fan
Case Fan: Cooler Master SickleFlow 69.7 CFM 120mm  Fan
Power Supply: OCZ ModXStream Pro 500W 80 PLUS Certified ATX12V / EPS12V Power Supply
Optical Drive: LG BH14NS40 Blu-Ray/DVD/CD Writer  
Operating System: Microsoft Windows 7 Home Premium Full (32/64-bit)  
Keyboard: Cooler Master CM Storm Trigger Wired Gaming Keyboard