Andikins, definately try to get a 9600 non pro or PRO [non SE or LE of course] over a 9200 if you can. 9200 cards are based on the Radeon 8500 and are somewhat old nowadays, they are not bad but with a 9200 you will probably find yourself wanting something a little more powerful pretty quickly. The 9600 otoh, for 20 pounds more will definately be powerful enough and last a year or two on new games at max detail.

heres a link to Tom's VGA chart with comparisons of 48 or so different cards at a few games -

http://www20.tomshardware.com/graphic/20031229/index.html

Since you have a Athlon 1.2 ghz then your mobo should have a AGP slot capable of 4x. So with that in mind im going to second amp_man and say go with a Radeon 9xxx card, best one for your price range would be a 9600 PRO since they are a steal for the price-performance that they give and it should be in your price range.
Plus if you search good enough you can get a 9600 pro with a Free half-life 2 coupon [when it ships].

But If your stuck with only PCI slots then i think it would be better to just upgrade your motherboard to one that has a AGP slot, then get a AGP card since they are better in most ways.

"Would a GeForce2 MX400 64MB DDR PCI be able to run the latest games (e.g. Max Payne 2, Deus Ex Invisible War)?

Woud this be true of all PCI graphics cards?"

No it wouldnt. In Deus ex 2's case it absoluely requires a card that can do hardware pixel shading ie- DirectX 8, and a GF2 cant do that, neither can a GF4 mx. If you dont have a DX8/9 card it just wont play. So your only option is to get a DX8/9 compliant card such as a Radeon 8500-and higher, or a Geforce 3 and up [not counting mx's, those wont play DX2].