I agree for G3258 as processor for best value/power ratio but it must be overclocked to 4.0 GHz so You have to find cheap mATX board with overclocking possibility not just any motherboard.
Topics I mentioned already describes old (ATI HD4xxx and older) and newer (AMD HD5xxx and newer) series. The most tested, cheap and reliable cards for groovymame will be HD4350 and HD5450. Starting from models HD6xxx (apart from HD6350 - modified 5450 actually) You will get only single analog output - only from DVI-I through converter DVI-VGA. For XP You will be forced to HD4xxx maximum, for 7x64 You can use almost any card up to HD7xxx (with some exceptions) though recommendations are cards from HD5xxx due easier setup process, faster modelines switching and proper speed with interlaced modes. The only downside is blurry picture (though less flickering) with interlaced resolutions.
As for RAM - 2 Gb for XP max (my PC worked in the past with 1 Gb perfectly) and 4 Gb for 7x64 max (again my PC works now with 2 Gb and everything is fine but I have only groovymame, nestopia, model2, vba-m, Taito Type X/X2 games, makaron and nullDC, I'm not sure about other emulators).
Due to the fact that XP support will be dropped by mame in some time is recommended to stick with 7x64 but for now under XP groovymame and other emulators works fine. Also speed of mame will be improved under windows 7x64.
PSU 400 W will be more than enough if You use HD4350 or HD5450 even You are going to overclock G3258 to 4,3 GHz. Tested on my configuration.
As for disks -
http://forum.arcadecontrols.com/index.php/topic,152314.0.htmlhttp://forum.arcadecontrols.com/index.php/topic,152131.0.htmlNOTE: You will not get proper performance for SSD disk under XP. If You are going to use SSD than You are force to windows 7x64 at least. Choosing between SSD or standard HDD is a matter of luxury (windows boot speed for example) and money. My whole set with selected games (not just mame as I posted) is about 100 Gb but You are going to Hyperspin route with all extra stuff 500 Gb will be better choice. If You are going to whole Hyperspin stuff then 3 TB might be required.
Please read all the topics I wrote previously to get more opinions and info.
Sorry for not replying Your mails but I really don't like wrote all the same for just one single person. Forum is better place for this.