Pfft, limewire only fucks up computers if you don't watch you download. Ive been using it for years with no problems. (AKA DONT DOWNLOAD THAT 300KB .EXE !!)
Right here. Either more ram, or a better processor. If you already have one of the better processor for your socket type, then you might want to look into upgrading the motherboard to get a faster processor. A lot of the time you can get a motherboard/cpu combo for 200 or so online, then just transfer over ram/video card (If its all capable) and bam.
I built myself a computer the beginning of this year.
asus motherboard
AMD Quad Core 2.5ghz processor
4 gigs of kingston ram
nvidia gforce 9800+ gtx 512 card
850watt antec power supply
1 terabyte hard drive
etc. and all brand new this ran me roughly 600$ at the beginning of this year. Fast computers are getting extremely cheap to buy/build.
You could probably build my computer for under 400 now, and my computer plays any game i throw at it just fine on high settings. (CS:Source mainly)