CORSAIR DOMINATOR 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 1066 (PC2 8500) Dual Channel Kit Desktop Memory - Retail
Or
G.SKILL PI Black 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory - Retail
Corsair is faster but more expensive, would have to wait longer to get another kit.
G.Skill is slower but less expensive and could afford another kit pretty soon.
Or
G.SKILL PI Black 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory - Retail
Corsair is faster but more expensive, would have to wait longer to get another kit.
G.Skill is slower but less expensive and could afford another kit pretty soon.