The OCZ Revo is the fastest thing on the planet.
Sequential Read: up to 540MB/s
Sequential Write: up to 490MB/s
It needs a 4x PCI-E slot, but it saves you needing to purchase a SATA 6Gb/s controller (which would also require a 4x slot).
Newegg has it for $345.00 after mail-in rebate, right under your budget.
It's a higher capacity than the Intel X25, and outperforms it (Intel reads up to 250MB/s, but only writes up to 70MB/s)http://www.newegg.com/Product/Product.aspx?Item=N82E16820227578