Today only, Fry's Electronics offers the Corsair XMS2 4GB PC2-6400 DDR2-800 240-Pin Dual Channel Desktop Memory Kit with two 2GB DIMMs, model no. TWIN2X4096-6400C5C, for $45.99. This $20 mail-in rebate cuts it to $25.99. Shipping adds $1.99, or opt for free in-store pickup. Even with shipping, that's $6 under our July mention and the least expensive 4GB PC2-6400 desktop RAM kit we've ever seen. (It's also a current price low by $20.) Sales tax is added where applicable. Features include 5-5-5-18 timings.
Update: It's now only available for in-store pickup.