maybe a 9600/9800XT... I don't think it's in your budget, but 6800 or 7800GS. Of course your won't play on high settings but... I've still got a 7800GS in my kids system and it plays all the games very nicely.
I have also just put in a extra 512 megabytes of ram but it did not seem to help the games much. It is not my processer as that runs at 2.8 ghz so i have assumed the games run poorly due to the video card i have which is a ATI Radeon with 32 megabytes of memery.
Sapphire X1300 256MB DDR AVIVO TV-Out/DVI (AGP) - Retail (GX-119-SP)
Price: Â£62.95 (Â£73.97 Including VAT at 17.5%)
If you can afford the little extra (I am guessing VAT is like Sales Tax here in the states). This would likely PLAY both games, but to be honest, with a budget of 80 pounds, oblivion is gonna have to stay at low settings... POSSIBLY medium.
I believe the impact of the sound adapter on the RAM in neglagable at best, but I can't be certain, I am not really a sound guy. I am not sure how much RAM an onboard sound solution would use compared to an expansion card.