Microsoft updates for XP are due to cease next April. You can still use the OS, but security flaws that are then discovered will not get patched. But if you have good antivirus / antimalware etc. the risks will be reduced.
XP will probably perform (a little) better than W7 with only 2GB of memory.
In the meantime XP Windows Updates may take an awful long time install each month (this is a common issue, see this thread).
Internet Explorer (e.g. v7 or 8) with XP will also be very slow and you would be well advised to install an alternative such as Chrome.
If you are not familiar with W7 there will be a bit of a learning curve.