Hmm... I'd get the cheapest card you can that'll match or exceed the performance of your older one. I'd not really dump a lot of money into an AGP system- unless you have plenty of money to spare- in which case I'd then say just get a new system.
I myself am stuck using a single core A64 3800 with an AGP 7800GS + 3.5GB DDR400 as I've run out of money to finish my i7 build. The 7800GS is no top performer, but it does the trick at near any res. Might have to drop the in-game settings to low in a lot of newer titles, but it'll at least play them.
I'd get the least expensive option you can for now, and put the rest of your money towards a complete system upgrade. You need to ditch AGP like a bad habbit.
It *is* a dead platform.
A lof of today's mainstream, and even some low-end CPUs, are incredibly fast for the price. For about $250 USD you can have a rockin' system... give or take some $$$.
I mentioned this in another thread, but not too long ago I build a rig for my older sister. It's sporting the PhenomII X2 550 BE, 3GB DDR2 800 RAM, and just using the HD3400 (I believe that's what it was ) that's integrated into the mobo. She doesn't game, but for stuff like browsing the web, making home movied, photo albums, music, etc it's blazing fast. Slap a nice video card in there and it'd be a rocking gaming rig. I built that for her at just over $300. Add maybe $100 for a good gaming card and you got just over $400. There's plenty you can do to shrink that price tag though.
Little over a year ago I build a rig for my older brother. It's rocking the Pentium E2140, 2GB DDR2 800, and one of my old 9600GTs. It's actually pretty darn fast. A rig like this is great at just day-to-day tasks, and with a card like the 9600GT it can game rather well also. Best part is, it was super cheap to build. I think under $300 including the card.
That's my advice.
AMD AthlonII X2 240 + Gigabyte 785G Mobo
+ 4GB (2x2GB) DDR2 800 G.Skill
for a grand total, shipped, of $208.90. That motherboard also has an integrated Radeon HD4200. That, even though it's integrated, would be a faster GPU than what you were previously running.
For a mere $208.90 you can have a brand new system. That Gigabyte mobo will also accept any curret AM3 processor from AMD. So if you ran into some money later on down the line, you could buy one of their AM3 PhenomII X4 CPUs and have a quad-core rig for cheap. Or, keep that AthlonII X2 and put in a better video card. GTX260 Core216s are going cheap, as are the 4870 1GB. Granted, all these prices are USD so I'm not sure what you could get over in London for a comparable price. But, it's worth looking into.