To those (still!) trying to answer the same question (it's now August, 2012), maybe I can help (and you do
not need to read this entire longish thread):
Just spent many hours looking for an answer to the same question. I'd say, based on Dell's website and several forums, that the "Presler" Pentium D P-945 3.4GHz 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 Core 2 Duo. That's probably true, but the best you can do in a 5150/E510 is a D-945.
The D-945 is dual core, btw - not single core as stated earlier in this thread. Pentium 4's are single core/HT CPU's, while Pentium D's are dual core/no HT CPU's. As recently as last year, several forum writers have said that the D-945 is relatively strong, by contemporary standards, in the E510. At
http://ark.intel.com/
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. You can also list all the CPU's that are compatible with an Intel motherboard's (MB's) 945G Express Chipset, and there are a lot, though they don't necessarily work with the E510's BIOS, right?. Your E510 CPU candidate search criteria needs to include:
■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.
**I still wonder a little about newer CPU's. Here, at
http://ark.intel.com/products/chipsets/8765
is the list I generated for CPU's that are compatible with an Intel MB's 945G Express Chipset. There are a lot of newer CPU's that fall within the parameters of an Intel PLGA775 MB. Just gotta' wonder if a couple of the higher up's might work in our Dell E510 BIOS.**
■Socket PLGA775.
■Power supply limit. My E510 has the standard 305W PS; the P4-630 draws 84W.
■■800MHz FSB. Dell spec's say motherboard is capable of 1,066 Mhz, and both 400MHz and 533Mhz RAM are supported. I looked at a CPU in the next higher "Extreme" CPU family that uses a 1,066MHz FSB, and saw that, otherwise, it meets our spec's. 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", 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 "Presler" Pentium D 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 did just upgrade 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). It's my library - on both OS's - and it's about to 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 it 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. I'm often suspicious of the goings-on within the Evil Empire. Maybe they want your xBox to be 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 neighboring university co-ed's on the way out to summer fun (maybe they were also on the way to 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.
Though there is that bigger PS/1,066MHz idea...