Upgrading a Dell Inspiron 1720

My girlfriend bought this laptop in 2007. At the time, it was a very good laptop with nice features. Needless to say that 8 years later it's old and lack power and memory. After reading a lot on the web about upgrading that laptop, I decided to give it a new life.
Original specs are as followed :
- CPU : Intel Core 2 Duo T7100, 1.8GHz, 800 MHz FSB
- GPU : NVidia GeForce 8500 GT, 256Mb
- RAM : 2Gb (2x1Mb) PC5300-333Mhz
- HD : 320Gb 5400RPM
- BIOS version : A02
- OS : Windows Vista 32bits

I started a few months ago with the easiest upgrade : RAM! Went from 2Gb to 4Mb! Much better already, no more swapping on the HD! But that was far from enough! The computer was still slow. Can't watch online streaming on Netflix for example because it lags too much, even at lower resolutions. Same thing on YouTube! WiFi connection would often take several minutes before you could finally browse the Internet. So, after much reading, I decided to go for the CPU upgrade! The T9500 seems to be the most powerfull CPU that the motherboard can handle, so T9500 is was! I found a used one for 85$ (with a 3 months guarantee). By the I received the CPU in the mail, I had updated many drivers and the BIOS to A09. Got the CPU and installed it. Wow! Big difference! Everything is much faster! I can now watch anything I want on Netflix and it streams wonderfully! WiFi connections to the router is so much faster now! Only a few seconds!
I'm quite happy so far! The next upgrade will be the hard drive! I plan on putting 2x500Gb 7200RPM HD inside this machine. The added space will be very welcomed. I ordered a HD caddy (for the 2nd HD) on Amazon and I'm still waiting for it. I got the hard drives yesterday. As soon as I get the caddy, I will proceed with the HD upgrade, but I will also upgrade the OS to Windows 7 Ultimate 64bits (clean install). I will let you know how it went in the next couple of weeks!