The most important piece of info is missing and that is the file sizes that you will be processing.
I also have Lightroom 2.5, but I'm using the v3 beta version + CS5. My tiff files can be anywhere between 100 and 160mb. I'm now running a 64bit version of Vista Ultimate with a quad core CPU and 8Gb ram which copes quite nicely, but prior to installing the 64bit version my pc would quite often crawl.
If you're using CS4 or CS5 you will need a decent graphics card to make full use of the software. Prior to CS4 it's not to important and onboard graphics should be ok, as will be the Novatech you list if your file sizes are a more respectable 40-50mb or less.
I would suggest though if possible you go for more ram which would mean using 64bit. Lightroom and CS4/5 come in both 32 & 64bit, so no problems there.