I hate waiting for my computer to do things. Swapping and paging at all? Add more RAM. CPU-starved while running multiple processes? Add more CPUs.
By and large, for most desktop purchases, the most economical and reliable system will have a Gigabyte 790 or 970 mboard (I've never had a Gigabyte mboard fail), a quad-core AMD PhenomII-965 CPU, the fastest RAM one can reasonably afford (likely 1333 or 1666), enough RAM so that swap is rarely used, and SATA 3GB/s or 6GB/s IFs with one or more single-platter drives (certain Hitachi 7Kx000 drives are good). All the data and programs one normally works with should fit in RAM with room to spare. Clamav and squid each want about 500MB RAM (program and disk cache) to work efficiently. Gnome and KDE have become memory hogs. After using KDE for about 10 years, I've switched to XFCE; it's a lot faster and I don't need the UI wiping my butt and nose.
With 8GiB 1066 RAM, I can build my firewall system from scratch (source packages predownloaded), keep all four CPUs pegged during compiles (GCC and Linux will keep them pegged for about 5 minutes each, and IO waits still remain around 0), and rarely see IO wait states greater than 0.1. Linux's disk caching really is that good. The whole thing built uses about 6GiB (both cached in RAM and stored on disk). The build takes about 100 minutes building either on iron or in a KVM session with write-back disk access. Using write-though caching raises IO wait states to (for me) unacceptable levels. I bought this system a few years ago and I'm *still* not sorry I did.
There is one very good reason to always have at least a dual-core CPU: X11. It's a pig; it slows down single-core system (has done for at least 15 years; my old dual PII-266 was noticeably faster/smoother than my PIII-800). A second core allows X11 to run independently of other apps.
Power-conscious? Use the on-demand cpu-freq governor and get idle CPUs down to the slowest frequency available (800MHz for the PhenomII-965). I don't know of a way to get it any better outside of actively controlling system voltages. Performance mode--all CPUs at max clock of 3.4GHz--uses 20-40W more power on my system. Turning CPUs off (disabling them via /proc) doesn't change power consumption. [I don't know why Intel and AMD don't use (the king of low power) Motorola's (Freescale's) trick of turning off the clock in the parts of the CPU that aren't being used at any given time.]
This is what I've done and will continue to do. It works for me®. Your mileage may vary®. Consult a doctor if you experience any side effects®.