To those trying to answer the same question (still trying - it's now August, 2012!), maybe I can help (and you only need to read about half of this essay):
Just spent many hours looking for an answer to the same question. It is pretty clear, based on Dell's website and many posts in several forums, that the "Presler" Pentium D-945 SL9QQ is the best you can do in a Dimension 5150/E510. One forum respondent said an E520 came later, and it can use a better/later Core 2 Duo. That's probably true, but the best you can do in a 5150/E510 is a D-945.
The Socket PLGA775 D-945 is dual core, and a few forum writers suggest it is close to, or equivalent to, to next Core 2 Duo generation of Intel desktop CPU's. Pentium 4's (P4's) are single-core/hyperthreaded (HT) CPU's, while Pentium D's (PD's) are dual-core/single-thread CPU's. While the families of the two chip types - P4's vs. PD's - overlap in time, the P4 was introduced before the PD. Not surprisingly, within the overall group of Socket 775 CPU's, the best PD eclipses the best P4. At
Intel has a nice list of ALL Intel desktop CPU's EVER, with major spec's. You pick CPU's you wish to compare, hit go, and compare. Your E510 CPU candidate search criteria needs to include:
(1) Pentium D family, or lower/older. I'm tempted, as others are, to go higher/newer. Forum writers say Dell's BIOS is a limiter, and newer chip families require different power management.
(2) Socket PLGA775.
(3) Power supply limit. My E510 has the standard 305W PS; my original P4-630 draws 84W. In related posts, a few writers suggest that Dell power supplies (PS) are robust, and have a safety margin of untapped capacity. I don't want to buy a new PS for this old machine, so I cut off my list of candidate CPU's at ~95W.
(4) 800MHz FSB. Dell spec's say the E510 motherboard (MB) FSB is capable of 1,066 Mhz, and both 400MHz and 533Mhz RAM are supported. I looked at the next higher "Extreme" CPU family - it uses a 1,066MHz FSB. If you're curious, brave, and don't have enough to do, I guess you could thumb your nose at all these Dell and forum "experts", get a bigger PS, and try a 1,066MHz FSB CPU. (Hmmm...)
My search ended up with a list of 4 top CPU upgrades, including one P4 and three PD's. Of these, the best CPU is the P-945 3.4GHz SL9QQ. I'm about to buy a used one on Amazon for $14.51, including shipping. Just gotta' see what the old E510 can do before I give it away to some low income folks. (Of course, there IS that bigger PS/1,066 FSB idea...)
A short story of affection for this obsolete old box: I just upgraded its Win7 x86 to Win8 x64 Release Preview. If you look carefully on the Dell support website, you'll see the E510 isn't supposed to be able to run Win7. It runs Win7 just fine (apparently Win8, too), has been an XP x86 Media Center Edition (MCE)/Win7 Ultimate dual-boot since Win7 (Ultimate) Release Candidate (RC) came out almost three years ago. It's my library - on both OS's - and I'm about to make it burp up it's content to my new 3.0TB NAS.
The E510 has also been a great media center/DVR-PVR - like lots of reviewers, I like MCE a lot. I find it interesting that MCE was an option, and cost extra with XP, then was free in most editions of Win7, and now, with Win8, MS is back to making it an option, and charging extra for it. Plus, while every reviewer I've ever read has said that MCE is great, and they love it - how often do you hear "love" with relationship to MS products? - MS never promotes it. Is MS, for some reason, trying to kill MCE in Win8? They say the strategy of making MCE an option makes Win8 less expensive for people who don't want it. Maybe. But more than a small majority of us desktop users - home AND business - handle more than a little "media". And especially given that the media mass and velocity is exploding, I just can't help but view the Evil Empire with a little suspicion. Maybe they want to sell you an xBox to act as your media center.
I (literally) picked up the E510 a few years ago when it was (literally) kicked to the curb with a virtually identical Dimension 5150 by a couple of neighboring university co-ed's as they headed out for the summer (maybe they were also heading into grownup life). MS was kind enough to convert my Win7 RC to a legit license. Perhaps I'm attached to this old box because I've done tons of stuff to it/with it, learned a lot with it, and it's never let me down, even when dropped and otherwise abused. And it's all been free.