would u consider overclocking? i dont know if the dell mobo would support overclocking (if its an xps im guessing it should...) but like lum said p4 have a lot of headroom for overclocking... u could pretty easily overclock to 3.4, the max i bet is close to 4 with proper cooling (probably water cooling to hit 4.0) but like i said 3.4 isnt very hard to hit... my firned has a 2.4 oced to 3.2, he could go to 3.4 but it isnt worth it for him since it doesnt have HT... he made it 800 mhz, u should make 400 easy. try that out and see if it offers any perforamnce increase.
make note though! if ur new to overclocking research first, make sure u dont overvolt anything, and take baby steps when overclocking. overclocking voids ur warrenty so u might wanna keep that in the back of ur head, although its hard to fry stuff if u take baby steps and dont overvolt. also u have an x850, u can use ATItool to overclock that beast, they overclock very well that would help increase perforamnce a decent amount (probably 500-700 marks on 3dmark05) just make sure it doest get too hot (ati tool shows the temp)...