Help with CPU upgrading

[whsck]

Hi, looking to upgrade my CPU+Heatsink. I have a budget of £60 ($90). Looking for somthing that doesnt bottleneck my graphics card.

CPU List (This is what my motherboard can support):
http://www.filehosting.org/file/details ... t_list.pdf

PC Specs

OS: Windows XP 32bit
Motherboard: Sapphire PC-AM2RS790G
Current CPU: DualCore AMD Athlon 64 X2, 2000 MHz (10 x 200) 3800+
Graphics Card: ATI Radeon HD 4870
Ram: 3gb Dual DDR2 SDRAM

Thank you.