...i just dismantled my main computer which was :
AMD X2 4400+
2Gb ram
1Tb hdd
ati hd3650??( cant be sure here)
It played mass effect 2 very nicely, also COD MW2....
SOOO...in making a good computer you must be sure to select components that wont bottleneck with anothers...
mainly...while CPU is important...Graphics card and RAM is much more for games....
big points to to look at:
GRAPHICS CARD: must have at least 256bit bus for 512MB ram...get away from 128bit bus ones...
1GB i would look for 512Bit bus (but this should be expensive...) neverthless! dont go for less then 256Bit BUS!
RAM: i normally dont care much about if its OCZ or Gskill, but i normally go for Kingston Hyperx or something...
check for low timings.....like this...you got loads of options of RAM...so settle for a frequency...i would go for about 1333? or even 1600...then check timings they go like this Cas:9-9-9-24 ...the lower you find the best....the lower the timing the more expensive it will be...4GB is enough for now...you may want to go for 8GB...
Motherboard: dont choose MSI or AsROCK....please...Gygabyte, asus, and some other might be good...go for a motherboard that will leave your with options for future...check CPU compatibility, RAM slots and speed.....
CPU: as i said i think its the less important stuff for gaming...may be for reencoding video, image manipulation...So a core i5 is mostly overkill....an i3 is enough as it is...
the bigger difference from an i3 to a i5 is that
core i3 are dual core with hyperthreading...
core i5 are quad core without hyperthreading.
Once again..for gaming dual core with hyperthreading is more then enough...whatever they say....
Power Supply: 650W with 80Plus certification is overkill already.
HDD: anything with high cache...32MB for 500GB, 64MB for 1TB and upwards.
get a nicely cooled case and stock CPU heatsink with cooler is enough.
this is my personal opinion from more then 6 years of working in a IT company and playing with computers since 10 years..im 24 now.
if you dont need to spend all the money...then dont, theres always something you want more...