1) 4Gb RAM is too low, 5xWoW under Vista are eating up to 5.5Gb in Orgrimmar or AV skirmish.
2) Dual core CPU is a no-no. Currently I have Q6600 @3.2GHz and sitting in Org eats 100% on all 4 cores. AV battles eat up to 80-90% CPU total load. Getting quad-core CPU is your first priority, more RAM is second.