Happy to help..... here are brief answers:
1. Lots of info in here if you search using the search box top left.... it targets this forum only.
2. Yikes on that price from Amazon. I only buy used processors, only from eBay, and I only get the latest sSpec code because Intel has built in improvements to the more recent version. For example, some of the xw8400 processors have 2 or 3 versions. For the E5345 processor you'd want the SLAEJ sSpec code, and by searching on eBay for SLAEJ you'll see that those are going for as low as $7.00-$8.00 USD including shipping each. You want two, I generally don't buy processors from China, and you can find great deals on these from good US sellers.
3. Hopefully you'll already have two heatsinks/fans. I mention in that other post there are 3 versions, and for the hotter processors you'll be using you want one of the two that has 3 total heat pipes.
4. Yes, you will see a very significant improvement by adding two of these faster quad cores. Your processor(s) are almost the slowest they ever sold with that workstation.
5. All your memory slots should be occupied by same size sticks for best memory performance. You're dealing with older slower memory in these workstations, so you must focus on getting the very best performance out of your memory. You've got a lot of room for improvement. 8 slots each filled with 2GB/stick should be your aim if 16GB would suffice, but if you need more then go for 8 slots x 4GB sticks. I only recommend HP memory because it has been "binned" for best performance. You already have 2 of your 8 slots filled, presumably with that HP memory..... 6 more used sticks off eBay will be cheap. Take a look at what you have already and you will likely see the part number I gave already to search eBay for. Ideally the brand of these HP sticks would be the same (such as all from Elpida or Nanya or Samsung) but that is not critical. Having all the same size sticks is pretty important.
6. The SSD is critical. I'll be dogmatic. Only get that Intel 320 series, and you want about 20% of it empty at all times. So, choose from the 160GB or the 300GB ones..... and you will be set for the long haul if you get the 300GB one and then have a SATA generation II documents HDD. I only buy those SSDs new or used off eBay, as with the HP memory.
7. I like the Noctua thermal paste, and it has been posted about how to do these upgrades here in the past.
Let us know if you need more help......