I'd say running Windows 7 with only 2GB of RAM on a Celeron 2.5GHz and a barely passable nVidia video card is likely to give you slower performance when doing things that require some horsepower, yes. Remember that the XP codebase is basically from 2001, and it was based on Windows 2000 and NT4, which goes back to 1996. XP can run very well on a Celeron 2.5GHz, and 2GB of RAM is enormous for work in XP. However, it is worth remembering that XP's bare minimum requirements were a 300MHz PC and 64MB of RAM (that's right, 64MB). Your system is ~8x faster in clock speed and has 32x the minimum RAM requirement. That would mean if you did the same to Windows 7's bare minimums (1GHz CPU, 512MB RAM), you'd have the equivalent of an 8GHz CPU with 16GB of RAM. Your PC, especially a Celeron E3300 2.5GHz, is pretty slow.
However, the real bottleneck is likely the GeForce 6800GS you're running - given that a LOT of the visuals on Windows 7 have been offloaded to the GPU, running a barely-capable DX9 GPU like the 6800GS, coupled with a Celeron, is probably holding you back. The RAM is likely not an issue at all, for what it's worth - 2GB is definitely a decent amount for almost everything, and 4-6GB is the sweet spot.