I am going to be upgrading a very old model just because I love the look of it and the keyboard. I'm not running tons of busy programs, but I want to speed it up a little and it's on the verge of crashing. I currently have 4gb RAM, 250gb HDD, Intel Core 2 Duo processor T5750 2.0 ghz, and run windows 7 pro 32 bit. It is 64 bit capable, so I am upgrading the os to the 64 bit version, upgrading to 256gb SSD, and will upgrade the CPU if possible or necessary to be able to also upgrade to 8gb RAM. I know it has 2 slots, so before I buy 2 4gb memory cards, I want to be sure I can actually use them. I ran a Crucial scan, but the os is still 32 bit, and I don't know if that influenced their report of a max 4gb RAM for this model.
The manual for the 1575 lists the maximum supported memory as 4 GB. Also hard to upgrade the CPU - there aren't a lot of Socket Ps to upgrade to even theoretically and Dell laptops aren't known for being good upgrade choices. Without any specific documentation (Dell doesn't even list the supported CPUs as least as far as I can find) and given the lack of significant upgrades, I don't think I'd worry too much about an upgrade.