hi dean
ive looked at the dell dimension 3100 and you have a problem since it only supports a pci-express x1 slot and a legacy pci bus slot.... and the card you have mentioned requires a pci-express x16 slot which your pc does not support
so what are you options... well not much to be honest,however there are some graphic cards that support the pci-express x1 slot... however you need to be aware because of the smaller slot size the performance will not be as good as pci-express x16 graphic cards
so what cards can you buy... well first theres the "Club 3D GeForce 210 PCI-E X1 Edition Graphics card"( http://www.trusthardware.co.uk/products.asp?partno=CGNX-212YLIX1 ) .. and this is also a low profile card that means it has a small bracket so it will fit in small form factor cases
also it will happily run with your 250w psu... however you will need to lower detail on most games and also the resolution....but it will run the games you stated without any problems
the next card i recommend is the "ATI FireMV 2250 Graphics card - low profile - 256 MB - DDR2 SDRAM"( http://www.scan.co.uk/products/256mb-ati-firemv-2250-pci-e-%28x1%29-gddr2-dms-59-s-video-sff-oem?utm_source=google+shopping&utm_medium=google+shopping ) and allthough this is not officially a gaming card its still a excellent card especially for a pci-express x1 card
in terms of performance this will run the games you mentioned and will be fine with your 250w psu... allthough your close to borderline in terms of power
another option would be to use the pci bus .. thats the long white slot found on all motherboards and you can also buy graphic cards supporting this slot....
one of the best pci based cards is the "Zotac ZT-62LEH2N-HSL GF 6200LE 512MB PCI DDR2 Silent Graphics Card"( http://www.amazon.co.uk/dp/B0051NIVN4/ref=asc_df_B0051NIVN45679787?smid=A3P5ROKL5A1OLE&tag=googlecouk06-21&linkCode=asn&creative=22206&creativeASIN=B0051NIVN4 ) ... this supports shader model 3.0 and will run games like team fortress 2 ect but again you will need to run at lower detail and resolution ....
you can also get the "PNY Geforce 8400GS / 512MB / PCI / Graphics Card"( http://www.play.com/PC/PCs/4-/9556293/PNY-Geforce-8400GS-512MB-PCI-Graphics-Card/Product.html ) which to be honest is probally your best overall bet..... this may be one of the slower geforce 8000 series card,however compared to other pci based cards its one of the fastest
and running games like team fortress 2 and terraria you will be able to run these smoothly at low to medium detail.... which is good for a pci based video card
all these will be fine with your dell dimension 3100,however i would have the recommend the last one namely the geforce 8400gs...... this is about the best pci card you can buy and will run with the 250w psu in your pc.....
i hope this helps dean,any questions let me know
good luck mate and happy xmas !