If you are going with building your own .. remember, for every part you buy there will be a warranty and support for that part...  If anything goes wrong you have to call the right support people to get it fixed....  It can drive you crazy if it's a compound problem with two different dealers...    I have heard that sometimes these support people play ping-pong with you.. ping tells you go to pong and pong tells you to go to ping,,,,  If you see what I mean... 
READ CAREFULLY YOUR WARRANTY>>>

There are web houses that sell combinations...   you can buy a few parts..ie.. (memory, CPU & MOBO)  combo, for a good price, under the same warranty...

The Motherboard is the most important part in a system so be ready to spend the most you can on a good one...  A motherboard is worth an extensive research...  unless you have the bucks, patience and time to deal with returning parts..  steer away from " the latest"  if it is untested in the public domain..  There are a lot of bells and whistles...  keep it simple...
Speaking of returns..  some web outfits are not very good in the process of returning Items...  I'm sure you know this..   Keep an eye out...  
READ CUSTOMER REVIEWS>>>>

There is also the CPU.. which one?  ADM or Intel.... ?  read reviews speak to people...! 

**Building your own system can be very satisfying and frugal... **

Lastly...  

The Graphics card...!  which one...?   One thing that can be said...  A good 3D system for modeling / rendering might make a poor gaming one.   If you don't care that much for games then you might save on some bucks....   You don't need the latest GC  to work in Brice...  Just plenty of fast memory...   If you are a gamer then you might want to spend more money...

BTW..  on the other hand.... if you have the money.. you can get a graphics card made for professional CAD system...    very fast renderings...   go poor even faster lol...

Last note...  there are magazines out there that build systems from scratch.... you can fallow one of their formulas..  since  they test and review the resulting "working" system...

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