As to games running "slowly" - any running application can create bottlenecks that may or may not concern a single particular component.
For example, when running Dwarf Frortress, Graphics card is completely irrelevant, CPU is what matters, but because the game is not threaded, you will want a CPU with faster clock frequency and fewer cores.
Arma 2, for example, has a huge texture pool and data is always streamed from memory to videocard memory, so the throughput of the videocard memory bus is important as well as size of video RAM, but the data can't fit into the VRAM anyway, you need a fast I/O too (SSD disk preferrably), or enough RAM for a RAMDISK.
These are just examples I use to point out that each application can have different demands you have to take into consideration when building a rig. People today tend to forget that CPU and GPU are sometimes not enough, with games having more and more graphical and sound data, disk and bus throughput maybe just as important.
Also, when buying a motherboard, look ahead and see what sockets it supports, what is a possible upgrade path for your CPUs to estimate motherboard longevity. I am for example stuck with 1156 chipset, so Core i5 750 is pretty much end of the line for me. If I want SandyBridge or IvyBridge, I have to toss out MOBO and CPU, which will be expensive.
As was said here, there are a lot of parameters to consider, some chipsets or combinations of chipset and components have odd bugs and glitches you most often find only AFTER you have bought them. I had 8800GT with bad memory for some time and could not run Dominions 3 at all. Other games worked fine.
