First of all, i'm quite surprised you are not having good results with this setup. Never tried any of these games yet but I know they are quite new.
Latest game I tried so far is Call of Duty and it works really great on my setup. I also have a GeForce FX 5600 (not the XT, don't know if it's better or not).
How many FPS do you get for considering this low?
Compared to Call of Duty, is Battlefield Vietnam more demanding on the system?
If you want to upgrade your system for cheap, go with AMD and you won't be disappointed. Find a board using the nforce2 chipset and put a 2400+ processor on it or better. Make sure your board support dual channel RAM memory for a couple more FPS. The MSI K7N2 Delta-L is quite cheap and very good.
You'll also be able to get a good price for your pentium and mobo on eBay so that will help you finding the funds for a good upgrade.
But hey, switching from a pentium 4 1.7 to let say an Athlon XP 2400+ platform will give you no more than around 15% improve in performance, according to tomshardware : http://www6.tomshardware.com/cpu/200...charts-23.html
At this point, I think that a better videocard will improve much more than a cpu switch.
But once again, let me say that i'm quite surprised that you can't run Battlefield Vietnam quite well with your system. Did you try Call of Duty?!