If you are looking for extra frames with a GPU upgrade in flight sim X you may be dissapointed. These games (flight sims ) are very CPU centric and realistically you would be better off trying to upgrade the whole system.
However first things first What Motherboard do you have, whay socket /chipset are we talking about?
Is it an AGP card you are after ?
As i first said the whole rig is weak (sorry ) You could use a faster CPU better GPU and more ram. Then the Power supply is probably quite low as well, Do you know what power supply is in it ?
Sorry to be so negative i will try to advise as best i can and i understand a budget is a budget, just dont want you pumping money into something that the rest of the sysyem will hold back.
Very cpu intensive game that my son was running as a slide show with athlon X2 4200+ and 9600GSO (was an upgrade for a 7900GS and no noticeable difference). It is the main reason we upgraded his machine to a quad core. In the meantime he played FS2004 which was playable.
I got this comp of a mate of mine is this what you need.
OS Name Microsoft Windows XP Home Edition
Version 5.1.2600 Service Pack 3 Build 2600
OS Manufacturer Microsoft Corporation
System Manufacturer VIA Technologies, Inc.
System Model PT800-8237
System Type X86-based PC
Processor x86 Family 15 Model 2 Stepping 9 GenuineIntel ~2806 Mhz
BIOS Version/Date Phoenix Technologies, LTD 6.00 PG, 25/12/2003
SMBIOS Version 2.3
Windows Directory F:\WINDOWS
System Directory F:\WINDOWS\system32
Boot Device \Device\HarddiskVolume2
Hardware Abstraction Layer Version = "5.1.2600.5512 (xpsp.080413-2111)"
Total Physical Memory 768.00 MB
Available Physical Memory 18.23 MB
Total Virtual Memory 2.00 GB
Available Virtual Memory 1.96 GB
Page File Space 1.83 GB
Page File F:\pagefile.sys