Hello,
I really don't think you will get that much of a performance improvement if you just swap out the cpu and gpu because the cpu's that fit LGA 775 are really old and don't offer for much compelling value today. If you need an upgrade, I would suggest going for LGA 1150 and a core i5-4690K (or non K), a GTX 960, asus mobo preferbaly and at least a 80+ bronze psu.
I'm sorry if this isn't what you are looking for but not getting a new computer would just be a wate of money as for just a little more money, you are getting much better performance and a machine that will last you a few more cpu and gpu generations while still being reasonably powerful to run the upcoming titles at 1080p. My recommendation is, in short, wait a little more, save some pennies here and there and you will not regret buying a completely new PC and you will only be happier.
A nice setup I would recommend would be:
Intel core i5-4690(K)
Asus Maximus VII Hero
Asus GTX 960 Strix or EVGA GTX 960 SSC ACX 2.0+
8 GB of a reputable brand RAM like Corsair, HyperX, Kingston...
WD Green 1 TB HDD (storage is really versatile and you can take a smaller capacity one if you won't use all of that space)
(totally optional but recommended) Cooler Master Hyper 212 EVO CPU cooler
A reputable brand PSU of at least 600W and 80+ bronze
Decent case with plenty of case fan and radiator mounts for eventual future upgrades and good airflow.
Hope this helped 😀
PS: Please note that this system build is customisable and you shouldn't hesitate to save a few bucks if you can but NEVER, really never buy hardware form a small company that no one heard of just for 10 or 15 dollars and trust me on this one especially for the PSU as this can lead to many problems