What are some upgrade possibilities for my crappy old PC?

According to HP, that mobo will support Core 2 Quad processors i.e.q6600, q9xxx, but they are now several generations and probably need to be purchased used, like on ebay.
Chipset Intel Q33 Express chipset X X
QuickSpecs HP Compaq dc5800 Business PC
Standard Features and Configurable Components
DA - 12863 Canada — Version 27 — October 28, 2009 Page 7
Processor and Speed*
One of the following
Intel Celeron Processors:
Intel Celeron 430 Processor (1.8-GHz, 512K L2 cache, 800-MHz FSB) X X
Intel Celeron 450 Processor (2.2-GHz, 512K L2 cache, 800-MHz FSB) X X
Intel Celeron Dual-Core Processors
Intel Celeron E1200 Processor (1.6-GHz, 512K L2 cache, 800-MHz FSB) X X
Intel Celeron E1400 Processor (2.0-GHz, 512K L2 cache, 800-MHz FSB) X X
Intel Celeron E1500 Processor (2.2-GHz, 512K L2 cache, 800-MHz FSB) X X
Intel Pentium Dual-Core Processors:
Intel Pentium E2180 Processor (2.0-GHz, 1-MB L2 cache, 800-MHz FSB) X X
Intel Pentium E2200 Processor (2.2-GHz, 1-MB L2 cache, 800-MHz FSB) X X
Intel Pentium E2220 Processor (2.4-GHz, 1-MB L2 cache, 800-MHz FSB) X X
Intel Pentium dual-core E5300 Processor (2.6-GHz, 2MB L2 cache, 800-MHz FSB) X X
Intel Pentium Dual Core E5400 Processor (2.70-GHz, 2MB L2 cache, 800-MHz FSB) X X
Intel Core 2 Duo Processors:
Intel Core 2 Duo E4600 Processor (2.40-GHz, 2 MB L2 cache, 800-MHz FSB) X X
Intel Core 2 Duo E6550 Processor (2.33-GHz, 4 MB L2 cache, 1333-MHz FSB) X X
Intel Core 2 Duo E6750 Processor (2.66-GHz, 4 MB L2 cache, 1333-MHz FSB) X X
Intel Core 2 Duo E7200 Processor (2.53-GHz, 3 MB L2 cache, 1066-MHz FSB) X X
Intel Core 2 Duo E7300 Processor (2.66-GHz, 3 MB L2 cache, 1066-MHz FSB) X X
Intel Core 2 Duo E7400 Processor (2.80-GHz, 3 MB L2 cache, 1066-MHz FSB) X X
Intel Core 2 Duo E7500 Processor (2.93-GHz, 3 MB L2 cache, 1066-MHz FSB) X X
Intel Core 2 Duo E8300 Processor (2.83-GHz, 6 MB L2 cache, 1333-MHz FSB) X X
Intel Core 2 Duo E8400 Processor (3.0-GHz, 6 MB L2 cache, 1333-MHz FSB) X X
Intel Core 2 Duo E8500 Processor (3.16-GHz, 6 MB L2 cache,1333-MHz FSB) X X
Intel Core 2 Duo E8600 Processor (3.33-GHz, 6 MB L2 Cache, 1333-MHz FSB) X X
Intel Core 2 Quad Processors:
Intel Core 2 Quad Q6700 Processor (2.66-GHz, 8 MB L2 cache, 1066-MHz FSB) X X
Intel Core 2 Quad Q8200 Processor (2.33-GHz, 4 MB L2 cache, 1333-MHz FSB) X X
Intel Core 2 Quad Q8300 Processor (2.50-GHz, 4 MB L2 cache, 1333-MHz FSB) X X
Intel Core 2 Quad Q9300 Processor (2.50-GHz, 6 MB L2 cache, 1333-MHz FSB) X X
Intel Core 2 Quad Q9400 Processor (2.66-GHz, 6 MB L2 cache, 1333-MHz FSB) X X

DDR2 is all you can use for memory, not sure the max on that system, wouldn't go higher than 8 (maybe only 4gb)

While it will take a PCIE graphics card, you are limited to a 'half height'(I believe your system is a SFF-Small Form Factor) card & by your PSU

It really is not worth putting to much money into.

Good news. Your machine will support any of the core 2 quads all the way up to the q9650. What are you using the pc for? Do you have the micro tower or the small form factor? The q9650 is roughly equivalent to a core i3, therefore, it is still a very capable cpu. I guarantee you will see a very large difference from your dual core. As for the ram, only ddr2 is compatible. I would just buy a 4 gb kit from amazon or wherever you shop and add it to the vacant slots on your pc. 6 gbs should be plenty, however, if you want more, your machine will support up to 16 gbs. As for the graphics card, I would go for a gtx 750 or 750 ti. Both use very little power so they will easily run on your 300 watt psu. I recommend making sure your bios is up to date before performing any upgrades.

Edit: The microtower is the only one that comes with a 300 watt psu. You will be able to upgrade to a full height graphics card.

I believe the Q9650 is a LOT more powerful than the i3, and even more so than one i5 that I have, this is a BEAST of a CPU, with a massive 12MB cache that no consumer based quad on the market has today. One has to step up to a 6 core to get above that, the lowest priced one (Haswell) has a 15MB cache.

That's just one of the things that makes a great CPU, looking on the Passmark site, the Q9650 blows away the i3's that I've seen, can have 30-40 Firefox or Chrome tabs open & the PSU fan doesn't start screaming, not does the heat rise a lot. Of course, am running a discrete GPU & that temp rises more, but still, for those with the Q9650, it's the BEST consumer based C2Q on the market.

There's 3 or so others that's really high dollar, and uses high wattage, though these are likely for hardcore gamers & for professionals who needs the best for their intensive work. This CPU can carry a load, just like a diesel.

Just purchased a Optiplex 780, and a XPS 8700 is my main PC, and have a few in between, this is by far my 2nd best PC. Even better than the quad core mobile i7-QM-3630 in my Samsung Series 7.

Cat

Evidently, the i3 has changed a lot in performance since my two versions, both 1st gen models, one a mobile version (i3-370M). First, there's the lack of Turbo Boost on today's i3's, secondly it's not a quad core CPU, though does have H/T available. Kind of like showing up at a gunfight with a pocket knife.:lol:

Finally, the i3 of any type won't fit in the socket of that MB, limited to Socket 775 CPU's. That's what the OP has to work with, therefore the Q9650 gets the nudge, if the tower PC version, there's two, maybe three Extreme models, yet these likely costs more than a new i3 would. Though would be way too much for the SFF to use, the CPU alone would be using over half of the PSU's max power output, and no room for the 3rd party cooler that would be needed.

I say it's reasonable to go with 8GB DDR2 RAM, and an Intel Core 2 Quad 9550-9650. My PC of the same model has 8GB RAM & Intel Core 2 Duo E8400 running at 3.0Ghz. BTW, am on the PC, so will post a shot at my specs. Note that I did at one time also had the OEM GPU from my XPS 8700 in here, using a low profile bracket, but when my Optiplex 780 arrived a couple of weeks back, dropped it in there, which has the Q9650. Speccy specs following.

http://speccy.piriform.com/results/zqDhlH2yVfzJsLG5w8Lrvn8

My main issue with the E8400 isn't that it's a totally bad CPU, it's that when the PC sits for 2-4 weeks and not booted, everything wants to update/upgrade at once, and this is having Windows Update set to notify only. The result is the PC will freeze, it's best to leave alone for 3-5 minutes, even with a SSD installed. Try opening a browser after boot, it'll surely hang. Not so with the Q9650 & likely the same with the Q9550, that's the purpose of the quad, to multitask with minimal slowdown & that applies even in the year 2016.

The Q9550/9650 aren't modern by today's standards, yet look at the massive (& crappy) mobile market we have today, an i7 (& not a true quad core) 16-18W (or sub-20W) CPU has zero chance at outperforming either of the legacy models (& some 1st through 3rd gen i5 & i7 mobiles). Though the desktop CPU's are a different story, the i7-6700K is a killer CPU, and the i7-4790K (my next upgrade) isn't far behind. The i3 comes close to neither, nor the i7-4770 in my XPS 8700 that'll soon be sitting in a box.

This Topic has some meaning to be because of one reason, I have the very same PC, in fact, quite a collection. Would love to hear from others if any of the Q9xxx series were performed and the outcome.

Cat