low benchmark score

[Julusp]

On your place, I would not upgrade the GPU at all, it's the least of your problems
Your configuration is Windows XP SP3, Phenom II X2, possibly 4GB ram and 3870.
Current issues:
Windows XP does not use more than 256MB of GPU ram so if you have 3870 with 1GB ram, 768MB of GPU ram will not be used.
Windows XP does not know how to use more than 3GB of system RAM (so if you have 4GB of ram, 1GB is wasted)
Phenom II X2 is not bad but possibly it is the HW bottleneck of your system.
Radeon 3870 is still ok for most current games (low-medium details), and should be more than sufficient for running FFXIV on standard details.

My recomendations:
Upgrade to Windows 7 which will allow you to use full potential of you GPU and RAM
Your mainboard supports Phenom II X6 (six-core) cpu, I would take a look to nearby stores how much it cost. Note: Bios update is needed before upgrade to X6 phenoms. (http://www.gigabyte.com/support-down....aspx?pid=3138 - list of supported CPU for your mainboard and which bios version is supporting it)

[wolfsspirit]

Upgrade to windows 7 64 bits and whenever possible get a quad processor , the rest of your system is ok

Myself , i play on a desktop computer intel quad core 2300 mhz , 2 Gb ram , an -->> AGP ATI HD3850 <<-- and windows 7 ,64 on high settings at a resolution of 1650x1080 and without any issues realy except for some lag once in a while when it's too busy .
the win7 64 bits upgrade is most important , secondary your processor , it may be possible to replace your dual processor with a quad.. i would look into that and save some money.