Just pick up a cheap gigabyte Radeon HD 2400XT (MAME doesn't need anything more , some other emulators may benefit from a faster card)
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125076Stay away from anything Nvidia graphics cards right now, basically they're all faulty, you can read why here:
http://www.theinquirer.net/gb/inquirer/news/2008/09/01/why-nvidia-chips-defectiveGet a good heatsink like the Sunbeam (Tuniq) CR-CCTF 120mm "Core Contact Freezer" (that way you can overclock your CPU and get the most out of it)
http://www.newegg.com/Product/Product.aspx?Item=N82E16835207004A good 7200rpm HDD is a must now for MAME, now the laser disc games are supported, they are very hard on disk I/O.
I suggest a Western Digital 300gb or larger 3.5" Sata drive, you are going to need the space for all the laser disc CHDs in the near future. (make sure you don't get any of their green drive models, they are good drive but have a slower average seak time.)
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136074Or if you want to go over the top on the hard drive preformance side or things and be really safe get a Western Digital VelociRaptor 10,000rpm drive.
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136260Vista 64bit is must to get the most out of MAME, XP Pro 64bit works but it's harder to find driver support for hardware.
and don't under estimate the effect a good power supply has on the reliablity, stability and longevity of a system, also a stable power supply is a must for stable overclocking.