Stutters are often related to not having updated motherboard drivers, specially chipset and SATA. Go to your motherboard vendor website and download the latest ones, don't rely on the ones Windows automatically install. Do the same with integrated sound and network drivers. But, in any way, Vista or 7 will give you a 20% to 50% boost of fps vs XP with multicore cpus.
Having different partitions for Windows, games and even the swap file (virtual memory), all defragmented, also helps with stuttering.