To identify bottlenecks in the cpu, I would check task manager to see if the load is hitting 100% for any length of time. If you have an Intel P4 or Core-family cpu, checking the FSB is also something to look for, as a lower setting would hobble your cpu. The i7 doesn't have an FSB, so this does not apply. You have to be aware that many cpus now employ energy saving modes such as SpeedStep which deliberately reduce the cpu multipliers to slow it down.
RAM can slow you down unless you use lower latency RAM, but this is mostly relevant to gamers. RAM that isn't fast enough to keep up with the cpu may lead to crashes as it is forced to run faster than designed. DDR2-800 is actually fast enough for most cpus, except for i7's.
Video cards can slow you down if you don't use one that is designed for you application. 3D modelling and CAD cards make use of OpenGL and benefit from cards designed for that. 3D games benefit from the latest 3D gaming cards listed on tomshardware, for example. You can usually tell the video card is the bottleneck in a game by the stuttering and low framerates - anything less than 30 is unplayable.
Hard disks are the slowest component of any pc and will benefit from a speedup; however, most 7200 rpm drives will do an adequate job these days. You can tell it is the bottleneck when your cpu is mostly idle and your hard disk is active for a long time, and optimizing it may require investigating what is running that takes so long (antivirus scanners are a typical cause). You can move up to a VelociRaptor (or even a cheaper early model Raptor) which spins at 10000 rpm, or get an Intel or OCZ Vertex SSD (other models currently have severe write lag problems).
