If you are simply interested in dedicating some cash for Seti Work Unit Production you could build an expandable computer farm. Three hundred dollars can get you about five thousand credits per day. I bought a bunch of the following on ebay. They produce about 2,000 Einstein Credits per day each at 70 Watts each. A single wall outlet would run 20 of them.
I hook them up to the internet two ways. First, I bought an eithernet switch at Radio Shack for about $30. I took the eithernet cable that comes out of the DSL modem and stuck it into one of the eight switch ports. Then ran additional cables from the remaining ports to my various computers and to my Lynksys wifi which now stands alone.
You can also get switches with more ports, or do like I did and just run one of the outlet cables into another switch. Its called a daisy chain. Some of my computers are in another room and I bought some of these to hook up via the wifi. I got a bunch of the 'dongles' below from this vendor for $7 delivered, so I don't know what it is with the new huge price. I suspect all of the similar items are good enough though.
IF THE LINKS ABOVE DON'T WORK OR GET OLD HERE IS PLAIN TEXT.
1). HP dc7800 small form factor (sff) 2.83ghz [intel core duo e8300] $100 delivered ebay. This is a very capable unit and was among the very best of the two core era PCs and cost about $1,000 each five years ago, I believe. I still use an old single core 2.4ghz pentium 4 as my primary computer, but might eventually migrate onto one of the HPs.
2) Wifi usb 'dongle' variable price, but I paid $7 each for mine and they work fine. If you don't know how to mess with winzip and drivers I would think twice about this option, unless it comes with an installation disk.
AND FINALLY CPU BENCHMARK.
You can compare CPUs at this site. For my purposes I found out my extra HP dc7800 2.0ghz machine had a score of 1000 and the 2.83ghz version was scored 2000. Both were $100 from ebay. The 2.8 is not twice as fast on the work units, but it is faster.