crack open your case and find out what ram you have exactly, and you get another stick of that. record the brand and the timing.
E6600 @ 3.0GHz | Gigabyte EP35-DS3R | Visiontek HD3870 @ 820/1225 | Mushkin Redline 2x2gb DDR2 @ 1066 5-5-5-12 | Corsair 550VX | WD 250gb | Seagate 320gb | Antec 900 | Thermaltake V1 | Zalman VF700