As a system builder I took on as a personal challenge to see what I could get working in the Small form factor C521.
Summary: As of January 2016 - So far I have a new 5600+ Athlon cpu overclocked with clock gen to 3.0ghz; a low profile NVidia 750 ti GPU running hdmi to display, and 10gb [yes 10gb RAM is possible, likely 16gb is too] by going to 64 bit Windows 10 pro on a SSD. I have the original DVD player and added a multi card reader for SD chips [if i hadn't put in an ssd I would use a 16 or 32gb sdhc class 10 card to leverage ready boost on a 7200 rpm hdd]. I have not upgraded the original psu but there are ones out there to get you to 400w, maybe even 450 gold. even with the upgrade the old psu works-Primarily due to the power sipping GPU tdp rating. lastly, I installed a 3 antenna pci port, wlan card to get 480mps for the wireless. I use a USB Bluetooth dongle and a usb hub on the front for accessory charging.
some would just say buy a new pc but my cost so far have been reasonable. RAM $24, GPU $100, 5600+ CPU $25, card reader $14. a PSU would run about $60-90 so we are talking about $160 to $250 in parts. The OS product key for windows 8 pro was $70. For now I have about $250 in it and have a very quick, updated HTPC that sits by my big screen and can stream anything at 1080p and play steam games fine.
yet to come - a psu upgrade to SilverStone Technology 450W SFX Form Factor 80 PLUS GOLD Full Modular Power Supply with +12V single rail, Active PFC (ST45SF-G). It is small enough to fit and and allow sata connections from dvd but I am not sure about the back mounting holes. there is a company that makes a plate with a 400w psu for $60 as an alternate solution. I am considering eliminating the dvd and just use an USB one when needed. This would free one of the precious 2 SATA connectors on the mobo to run another larger HDD that could store movies, games, pics and music. I considered adding a case fan but it would have to be a 80mm one attacked to back grate but i think it would do little good in lowering temps. it still has the original CPU cooler so changing it to a better low profile one could eliminate the CPU black shroud and allow me to use the 92mm fan that is there to change to a better and more quiet one for case ventilation. I have a copper fin southbridge chip heat dissipater that would fit but so far the temp don't call for it.
OS: i went from XP Pro 32 bit sp3 to win 8 pro [clean install], then to 8.1 free through a MS store update, and then to win 10 pro free using the free upgrade option. I wrote down the product key using magicjellybean and then did a clean install of win 10 pro from the 'create media option' that allows you to burn a full OS iso that I burned to a dvd. Now i have a clean win 10 pro 64 bit os.
Trickiest was the RAM, i found off ebay 16gb of pc2-4200 [4x4gb] ddr2 ram for $25 so I gave it a try. all 4 installed wouldn't post so after trying a few combos i installed as you see in speccy report. I know this is choking my frequency of the 1gb cards in the other 2 slots but it give me a stable total of 10gb. Getting it stable with all 4x4gb installed I think is possible with a OS reset using win 10. I know the 64bit OS will take 16gb. All specs say 4gb is the max but they are old and wrong. The right RAM is 16GB KIT (4 x 4GB) Intel Motherboard DG43GT DG45ID. DIMM DDR2 NON-ECC PC2-6400 800MHz RAM Memory. Genuine A-Tech Brand, but they are all over $300 since they are server RAM so ultimately it's not cost effective. there is also something special about the AMD chip and the memory. i got this http://www.ebay.com/itm/351397763184?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT and the high density matters.
If there is interest, I can elaborate on part upgrades further as responses dictate. cheers
