Upgrade Z505S

I've a Sony PCG-Z505S. Would I be able to use a new motherboard, processor, hard drive to get it up to today's standard? The magnesium case and screen resolution are great but it's slow and running Windows 98.

 

This laptop was manufactured in 1999, it's 12 yrs old. There is no way that you can get this laptop up today's standards.
The only upgrade i would suggest is may be 256GB and upgrading to XP. I would only do the HD upgrade if you need more space or if the old drives defective. Still, i would be hesistant spending money on this very old laptop. Sony states that 192GB is max ram for this model. 64MB is soldered and you have 1 available memory slot.
http://www.docs.sony.com/release/specs/PCGZ505S_mksp.pdf

This site suggests that some people have upgraded to 256GB. You could try it. Even still 256GB is not great and may be consider keeping this as a second computer if it isn't already and buying a new one.
Sony Vaio Memory Upgrade

Sony Vaio Hard Drive Upgrade

 

I have a Z505JS in storage, too, it's really not worth the money to update it. You can only upgrade RAM to 256MB if you already have a 128MB soldered in and put another 128MB in the empty slot. HDD can be upgraded, but BIOS doesn't allow above 64GB drive and it's a PATA, so the best you could do is a 7200RPM 60GB or 32GB SSD.

None of these really provide performance to today's standard, though, with the exception of the extremely rare and expensive PATA 32GB SSD. All these parts are hard to find brand new, and even used one or ebay their price really are not competitive against current generation component.

Sorry, pal. I like my Z505, too, but the internals are only worth updating as a hobby to crazily mod a laptop or in a post apocalypse world where all the new ones are destroyed.