There are 3 areas I’d like to cover as upgrades that either improve performance or just ones that I’ve done myself that I’d like to share. There are a lot of question marks about which CPUs or Wireless adapters are compatible. I will tell you what I know for fact will work and what the results of those upgrades were.
CPU / Processor
I replaced the stock Intel Pentium T3400 processor (2.16 GHz, 1MB L2, 667 MHz, 65nm) with an Intel Core2 Duo T9550 processor (2.66 GHz, 6MB L2, 1066 MHz, 45nm) for about $25 off Ebay.com. Replace is straight forward; remove the bottom access panel, loosen the heatsink screws and remove the heatsink, unlock the CPU from the socket, and swap it out for the new one. Requires a Phillips screwdriver. After booting for the first time, your Windows system will tell you to reboot to install processor drivers. My Windows score for the CPU went from 5.4 to 6.3. No heat issues at all, perform is noticeably better.
I bought my Dell second-hand for less than $100 and it came with a… questionable power adapter. It seemed to only charge the laptop sometimes and even that became less frequent with time. Finally I did some research and found that my adapter was simply the wrong type. It was close but not exact and the tip was not shaped correctly (it was round when it needed to be octagonal). Replaced from Ebay.com for $10. Now my laptop charges just fine, every time. If you find that replacing your power cord did not change the situation, try another outlet in another room, and if that still fails then your DC power jack is probably dislodged/broken. This part is extremely cheap (<$5) but it is definitely an intermediate/advanced technical challenge to do the labor to replace it. If you’re not comfortable doing this, bring it to a professional to do the actual replacement.
I wrote a more comprehensive article on this particular upgrade and that can be found here: DELL INSPIRON 1545 WIRELESS ADAPTER UPGRADE (802.11AGN 5 GHZ WIFI + BLUETOOTH 4.0). Basically, the summary is that I found the stock Dell Wireless 1397 WLAN Mini-Card adapter to be inadequate for some (most) tasks such as connecting to a 5 GHz SSID. I replaced the stock adapter with an Intel Centrino Advanced-N 6235 adapter which gave me 5 GHz connectivity as well as Bluetooth 4.0 for around $25 off Ebay. Installation procedure consists of removing the bottom access panel and swapping out the exposed wireless adapter. Very cheap and easy upgrade that yields some great performance.