First of all, you replaced your 8800GTX with a card that is virtually about the same!
The 8800 Ultra was a bit faster, and the 9800GTX was about the same, then the 9800GTX+ came out and was just ahead of that. Nvidia then decide to rename it the GTS250 to get rid of ther old cards.
You can see the temperature of the video card in the NVIDA control panel, or by using several temp monitoring software programs such as "SpeedFan"
You might try overclocking the card a bit, try EVGA's precison overclockign utility, you might want to turn up the fan speed a bit too, just don't go crazy (No more then 50-75%) or it will be realy loud.
http://downloads.guru3d.com/Videocards----Overclocking-&-Tweaking_c13.html
Guru of 3-D has tons of programs and utilities you might like..at minumum i'd download 3DMARK 06 and see what kind of baseline score it gives you and compare it to other similar systems.
Before that, go to PCPITSTOP.com and run the "Overdrive" test and see if it finds any problems....you should score around 1000-2000 points (Click on the performce tab when the test is done)
http://www.pcpitstop.com/betapit/
WOW should run fast on that system, but if you are running the new one (Lynchking?) then I have read TONS of complaints about it, the software is not every optimised even on a faster system then yours, though increasing your CPU speed by Overclocking it might help some from what I remeber, it likes a very fast CPU!
Let me see what I can dig up on that, back in a bit.
System requirements are pretty low, reccomended requirements are way below what you have, so it SHOULD run great!
PC System Requirements
OS: Windows XP / Windows Vista (with latest Service Packs)
Processor:
Minimum: Intel Pentium 4 1.3 GHz or AMD Athlon XP 1500+
Recommended:
Dual-core processor, such as the Intel Pentium D or AMD Athlon 64 X2
Memory:Minimum: 512 MB RAM (1GB for Vista users)
Recommended: 1 GB RAM (2 GB for Vista users)
Video:Minimum: 3D graphics processor with Hardware Transform and Lighting with 32 MB VRAM Such as an ATI Radeon 7200 or NVIDIA GeForce 2 class card or better
Recommended:
3D graphics processor with Vertex and Pixel Shader capability with 128 MB VRAM Such as an ATI Radeon X1600 or NVIDIA GeForce 7600 GT class card or better
You might want to read up over at the WOW forums, they have a sticky on performance tips:
http://forums.worldofwarcraft.com/thread.html?topicId=10971909544&sid=1
I thought this was intresting and may help:
NVIDIA cards
Try turning off Transparency Antialiasing and Ambient Occlusion in the NVIDIA Control Panel.
NVIDIA PhysX drivers may have odd interactions with the video card's performance. If that is installed, try uninstalling or disabling the PhysX drivers.
NVIDIA GTX 200 series video cards may stay in power saving mode, giving you lower performance. Please check your power savings settings. Keep in mind that we do not support third party software but guides like these are available -
http://www.overclock.net/nvidia/423880-howto-prevent-gtx-200-series-downclocking
Here is the general tech support forum, see if you find other intresting tips there.
http://forums.worldofwarcraft.com/board.html?forumId=11110&sid=1
EDIT: what are your plans for the old 8800GTX?